Investments - Amortized Cost and Estimated Fair Values of Company's Fixed Maturity Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 788,190	$ 790,866
Gross Unrealized Gains	17,670	3,454
Gross Unrealized Losses Non-OTTI	(8,218)	(25,249)
Fair Value	797,642	769,071
U.S. Government and Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	19,955	19,771
Gross Unrealized Gains	324	8
Gross Unrealized Losses Non-OTTI	(50)	(458)
Fair Value	20,229	19,321
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	40,338	40,503
Gross Unrealized Gains	186	48
Gross Unrealized Losses Non-OTTI	(574)	(1,493)
Fair Value	39,950	39,058
Corporate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	727,897	730,592
Gross Unrealized Gains	17,160	3,398
Gross Unrealized Losses Non-OTTI	(7,594)	(23,298)
Fair Value	$ 737,463	$ 710,692