Investments - Amortized Cost and Fair Values of Fixed Maturity Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 770,655	$ 790,866
Gross Unrealized Gains	39,043	3,454
Gross Unrealized Losses Non-OTTI	(1,519)	(25,249)
Fair Value	808,179	769,071
U.S. government and agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	19,886	19,771
Gross Unrealized Gains	1,068	8
Gross Unrealized Losses Non-OTTI	(62)	(458)
Fair Value	20,892	19,321
Non-U.S. government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	33,233	40,503
Gross Unrealized Gains	1,193	48
Gross Unrealized Losses Non-OTTI	0	(1,493)
Fair Value	34,426	39,058
Corporate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	717,536	730,592
Gross Unrealized Gains	36,782	3,398
Gross Unrealized Losses Non-OTTI	(1,457)	(23,298)
Fair Value	$ 752,861	$ 710,692