Accumulated Other Comprehensive Income - Components of AOCI (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
AOCI [Roll Forward]
Beginning Balance	$ 2,946,188	$ 3,607,690
Ending Balance	3,123,533	2,946,188
Unrealized gains (losses) on available-for-sale securities, net
AOCI [Roll Forward]
Beginning Balance	773,791	1,046,680
OCI before reclassifications	(88,527)	(263,049)
Amounts reclassified from AOCI	32,842	(9,840)
Net current period OCI	(55,685)	(272,889)
Ending Balance	718,106	773,791
Total Accumulated OCI Balance
AOCI [Roll Forward]
Beginning Balance	773,791	1,046,680
OCI before reclassifications	(88,527)	(263,049)
Amounts reclassified from AOCI	32,842	(9,840)
Net current period OCI	(55,685)	(272,889)
Ending Balance	$ 718,106	$ 773,791