Investments - Credit Ratings of Fixed Maturity Investments, Held-to-Maturity (Detail) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 770,655	$ 790,866
Fair Value	808,179	769,071
Non-Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 0
Marketable Securities Member | Portfolio Concentration Risk Member
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	100.00%
Marketable Securities Member | Portfolio Concentration Risk Member | Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	13.00%
Marketable Securities Member | Credit Rating Concentration Risk | Non-Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	3.70%
Held-to-maturity Securities | Credit Rating Concentration Risk | Non-Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	0.00%
AAA Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 61,291
AAA Rated | Marketable Securities Member | Credit Rating Concentration Risk | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	31.40%
AAA Rated | Held-to-maturity Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	8.30%
AA Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 124,838
AA Rated | Marketable Securities Member | Credit Rating Concentration Risk | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	15.10%
AA Rated | Held-to-maturity Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	15.40%
A Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 513,409
A Rated | Marketable Securities Member | Credit Rating Concentration Risk | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	34.40%
A Rated | Held-to-maturity Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	64.70%
BBB Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 108,522
BBB Rated | Marketable Securities Member | Credit Rating Concentration Risk | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	14.70%
BBB Rated | Held-to-maturity Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	11.50%
Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 119
Not Rated | Marketable Securities Member | Credit Rating Concentration Risk
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	0.70%
Not Rated | Held-to-maturity Securities | Credit Rating Concentration Risk
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	0.10%
U.S. government and agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 19,886	19,771
Fair Value	20,892	19,321
U.S. government and agency | Non-Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 0
U.S. government and agency | Marketable Securities Member | Portfolio Concentration Risk Member | Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	0.30%
U.S. government and agency | AAA Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 19,491
U.S. government and agency | AA Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,378
U.S. government and agency | A Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
U.S. government and agency | BBB Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
U.S. government and agency | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23
Non-U.S. government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	33,233	40,503
Fair Value	34,426	39,058
Non-U.S. government | Non-Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 0
Non-U.S. government | Marketable Securities Member | Portfolio Concentration Risk Member | Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	0.60%
Non-U.S. government | AAA Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 0
Non-U.S. government | AA Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,446
Non-U.S. government | A Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	24,980
Non-U.S. government | BBB Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Non-U.S. government | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost
Corporate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	717,536	730,592
Fair Value	752,861	$ 710,692
Corporate | Non-Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 0
Corporate | Marketable Securities Member | Portfolio Concentration Risk Member | Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
% of Total Investments	12.10%
Corporate | AAA Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 41,800
Corporate | AA Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	114,014
Corporate | A Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	488,429
Corporate | BBB Rated | Investment Grade
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	108,522
Corporate | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 96