Investments - Credit Ratings of Fixed Maturity and Short-Term Investments, Trading (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost, total	$ 6,248,488
Fair Value, total	$ 6,149,516
% of Total Investments, total	100.00%
Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	3.80%
Amortized Cost and Fair Value, total	$ 233,968
AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	30.20%
Amortized Cost and Fair Value, total	$ 1,854,968
AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	16.80%
Amortized Cost and Fair Value, total	$ 1,034,335
A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	34.20%
Amortized Cost and Fair Value, total	$ 2,101,161
BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	14.30%
Amortized Cost and Fair Value, total	$ 881,003
Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	0.70%
Amortized Cost and Fair Value, total	$ 44,081
Fixed maturity and short-term investments, at fair value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	5,457,622
Fair Value	$ 5,380,445
% of Total Investments	87.50%
Fixed maturity and short-term investments, at fair value | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 233,968
% of Total Investments	4.30%
Fixed maturity and short-term investments, at fair value | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 1,789,386
% of Total Investments	33.30%
Fixed maturity and short-term investments, at fair value | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 874,792
% of Total Investments	16.30%
Fixed maturity and short-term investments, at fair value | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 1,637,068
% of Total Investments	30.40%
Fixed maturity and short-term investments, at fair value | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 801,329
% of Total Investments	14.90%
Fixed maturity and short-term investments, at fair value | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 43,902
% of Total Investments	0.80%
Fixed maturity and short-term investments, at fair value | U.S. Government and Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 776,614
Fair Value	$ 775,798
% of Total Investments	12.60%
Fixed maturity and short-term investments, at fair value | U.S. Government and Agency | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 0
Fixed maturity and short-term investments, at fair value | U.S. Government and Agency | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	760,063
Fixed maturity and short-term investments, at fair value | U.S. Government and Agency | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	8,588
Fixed maturity and short-term investments, at fair value | U.S. Government and Agency | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	7,147
Fixed maturity and short-term investments, at fair value | U.S. Government and Agency | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity and short-term investments, at fair value | U.S. Government and Agency | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity and short-term investments, at fair value | Non-U.S. Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	457,518
Fair Value	$ 444,786
% of Total Investments	7.20%
Fixed maturity and short-term investments, at fair value | Non-U.S. Government | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 28,791
Fixed maturity and short-term investments, at fair value | Non-U.S. Government | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	135,419
Fixed maturity and short-term investments, at fair value | Non-U.S. Government | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	212,238
Fixed maturity and short-term investments, at fair value | Non-U.S. Government | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	48,450
Fixed maturity and short-term investments, at fair value | Non-U.S. Government | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	19,888
Fixed maturity and short-term investments, at fair value | Non-U.S. Government | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity and short-term investments, at fair value | Corporate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	2,850,063
Fair Value	$ 2,812,066
% of Total Investments	45.70%
Fixed maturity and short-term investments, at fair value | Corporate | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 135,051
Fixed maturity and short-term investments, at fair value | Corporate | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	157,809
Fixed maturity and short-term investments, at fair value | Corporate | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	451,247
Fixed maturity and short-term investments, at fair value | Corporate | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	1,360,378
Fixed maturity and short-term investments, at fair value | Corporate | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	703,877
Fixed maturity and short-term investments, at fair value | Corporate | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	3,704
Fixed maturity and short-term investments, at fair value | Municipal Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	28,323
Fair Value	$ 28,174
% of Total Investments	0.50%
Fixed maturity and short-term investments, at fair value | Municipal Bonds | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 0
Fixed maturity and short-term investments, at fair value | Municipal Bonds | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	4,671
Fixed maturity and short-term investments, at fair value | Municipal Bonds | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	10,815
Fixed maturity and short-term investments, at fair value | Municipal Bonds | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	12,688
Fixed maturity and short-term investments, at fair value | Municipal Bonds | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity and short-term investments, at fair value | Municipal Bonds | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity and short-term investments, at fair value | Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	394,675
Fair Value	$ 391,962
% of Total Investments	6.40%
Fixed maturity and short-term investments, at fair value | Residential Mortgage-Backed | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 1,149
Fixed maturity and short-term investments, at fair value | Residential Mortgage-Backed | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	377,011
Fixed maturity and short-term investments, at fair value | Residential Mortgage-Backed | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	992
Fixed maturity and short-term investments, at fair value | Residential Mortgage-Backed | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	7,369
Fixed maturity and short-term investments, at fair value | Residential Mortgage-Backed | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	5,437
Fixed maturity and short-term investments, at fair value | Residential Mortgage-Backed | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	4
Fixed maturity and short-term investments, at fair value | Commercial Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	286,373
Fair Value	$ 284,575
% of Total Investments	4.60%
Fixed maturity and short-term investments, at fair value | Commercial Mortgage-Backed | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 3,357
Fixed maturity and short-term investments, at fair value | Commercial Mortgage-Backed | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	114,380
Fixed maturity and short-term investments, at fair value | Commercial Mortgage-Backed | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	39,976
Fixed maturity and short-term investments, at fair value | Commercial Mortgage-Backed | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	61,679
Fixed maturity and short-term investments, at fair value | Commercial Mortgage-Backed | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	25,173
Fixed maturity and short-term investments, at fair value | Commercial Mortgage-Backed | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	40,010
Fixed maturity and short-term investments, at fair value | Asset-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	664,056
Fair Value	$ 643,084
% of Total Investments	10.50%
Fixed maturity and short-term investments, at fair value | Asset-Backed | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 65,620
Fixed maturity and short-term investments, at fair value | Asset-Backed | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	240,033
Fixed maturity and short-term investments, at fair value | Asset-Backed | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	150,936
Fixed maturity and short-term investments, at fair value | Asset-Backed | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	139,357
Fixed maturity and short-term investments, at fair value | Asset-Backed | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	46,954
Fixed maturity and short-term investments, at fair value | Asset-Backed | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	184
Fixed maturity investments, at amortized cost
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	790,866
Fair Value	$ 769,071
% of Total Investments	12.50%
Fixed maturity investments, at amortized cost | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 0
% of total fair value	0.00%
Fixed maturity investments, at amortized cost | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 65,582
% of total fair value	8.50%
Fixed maturity investments, at amortized cost | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 159,543
% of total fair value	20.70%
Fixed maturity investments, at amortized cost | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 464,093
% of total fair value	60.30%
Fixed maturity investments, at amortized cost | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 79,674
% of total fair value	10.40%
Fixed maturity investments, at amortized cost | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 179
% of total fair value	0.10%
Fixed maturity investments, at amortized cost | U.S. Government and Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 19,771
Fair Value	$ 19,321
% of Total Investments	0.30%
Fixed maturity investments, at amortized cost | U.S. Government and Agency | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 0
Fixed maturity investments, at amortized cost | U.S. Government and Agency | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	19,288
Fixed maturity investments, at amortized cost | U.S. Government and Agency | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity investments, at amortized cost | U.S. Government and Agency | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity investments, at amortized cost | U.S. Government and Agency | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity investments, at amortized cost | U.S. Government and Agency | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	33
Fixed maturity investments, at amortized cost | Non-U.S. Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	40,503
Fair Value	$ 39,058
% of Total Investments	0.60%
Fixed maturity investments, at amortized cost | Non-U.S. Government | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 0
Fixed maturity investments, at amortized cost | Non-U.S. Government | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity investments, at amortized cost | Non-U.S. Government | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	31,743
Fixed maturity investments, at amortized cost | Non-U.S. Government | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	7,315
Fixed maturity investments, at amortized cost | Non-U.S. Government | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity investments, at amortized cost | Non-U.S. Government | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	0
Fixed maturity investments, at amortized cost | Corporate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	730,592
Fair Value	$ 710,692
% of Total Investments	11.60%
Fixed maturity investments, at amortized cost | Corporate | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 0
Fixed maturity investments, at amortized cost | Corporate | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	46,294
Fixed maturity investments, at amortized cost | Corporate | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	127,800
Fixed maturity investments, at amortized cost | Corporate | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	456,778
Fixed maturity investments, at amortized cost | Corporate | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	79,674
Fixed maturity investments, at amortized cost | Corporate | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 146