Investments - Credit Ratings of Fixed Maturity and Short-Term Investments, Trading (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost, total	$ 6,299,814
Fair Value, total	$ 6,289,057
% 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	$ 237,407
AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	31.70%
Amortized Cost and Fair Value, total	$ 1,995,364
AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	15.10%
Amortized Cost and Fair Value, total	$ 950,807
A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	34.10%
Amortized Cost and Fair Value, total	$ 2,146,880
BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	14.60%
Amortized Cost and Fair Value, total	$ 917,840
Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
% of Total Investments, total	0.70%
Amortized Cost and Fair Value, total	$ 40,759
Fixed maturity and short-term investments, at fair value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	5,511,624
Fair Value	$ 5,491,415
% of Total Investments	87.40%
Fixed maturity and short-term investments, at fair value | Non- Investment Grade
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 237,407
% 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	$ 1,928,840
% of Total Investments	35.10%
Fixed maturity and short-term investments, at fair value | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 828,266
% of Total Investments	15.10%
Fixed maturity and short-term investments, at fair value | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 1,630,074
% of Total Investments	29.70%
Fixed maturity and short-term investments, at fair value | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 826,216
% of Total Investments	15.10%
Fixed maturity and short-term investments, at fair value | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 40,612
% of Total Investments	0.70%
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	$ 809,975
Fair Value	$ 819,204
% of Total Investments	13.00%
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	812,731
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	6,473
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	0
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	444,600
Fair Value	$ 439,878
% of Total Investments	7.00%
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	$ 21,530
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	132,415
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	198,483
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	65,524
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	21,926
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	2,858,804
Fair Value	$ 2,861,943
% of Total Investments	45.60%
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	$ 155,018
Fixed maturity and short-term investments, at fair value | Corporate | AAA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	182,580
Fixed maturity and short-term investments, at fair value | Corporate | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	443,654
Fixed maturity and short-term investments, at fair value | Corporate | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	1,345,139
Fixed maturity and short-term investments, at fair value | Corporate | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	733,329
Fixed maturity and short-term investments, at fair value | Corporate | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	2,223
Fixed maturity and short-term investments, at fair value | Municipal Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	17,209
Fair Value	$ 17,421
% of Total Investments	0.30%
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	5,117
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	8,835
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	3,469
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	450,645
Fair Value	$ 451,361
% of Total Investments	7.20%
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	$ 1,040
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	438,487
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	471
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	8,916
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	2,443
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	278,868
Fair Value	$ 276,134
% of Total Investments	4.40%
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	$ 3,266
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	119,984
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	34,359
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	61,460
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	18,866
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	38,199
Fixed maturity and short-term investments, at fair value | Asset-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	651,523
Fair Value	$ 625,474
% of Total Investments	9.90%
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	$ 56,553
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	237,526
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	135,991
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	145,566
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	49,652
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	186
Fixed maturity investments, at amortized cost
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	788,190
Fair Value	$ 797,642
% of Total Investments	12.60%
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	$ 66,524
% of total fair value	8.30%
Fixed maturity investments, at amortized cost | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 122,541
% of total fair value	15.40%
Fixed maturity investments, at amortized cost | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 516,806
% of total fair value	64.70%
Fixed maturity investments, at amortized cost | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 91,624
% of total fair value	11.50%
Fixed maturity investments, at amortized cost | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 147
% 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	$ 19,955
Fair Value	$ 20,229
% 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	18,826
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	1,376
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	27
Fixed maturity investments, at amortized cost | Non-U.S. Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	40,338
Fair Value	$ 39,950
% 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	11,320
Fixed maturity investments, at amortized cost | Non-U.S. Government | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	28,630
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	727,897
Fair Value	$ 737,463
% of Total Investments	11.70%
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	47,698
Fixed maturity investments, at amortized cost | Corporate | AA Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	109,845
Fixed maturity investments, at amortized cost | Corporate | A Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	488,176
Fixed maturity investments, at amortized cost | Corporate | BBB Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	91,624
Fixed maturity investments, at amortized cost | Corporate | Not Rated
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amortized Cost	$ 120