Accumulated Other Comprehensive Income - Components of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
AOCI [Roll Forward]
Beginning Balance			$ 2,946,188	$ 3,607,690
Other comprehensive income (loss)	$ 64,908	$ (100,700)	133,382	(141,873)
Ending Balance	2,963,080	3,440,065	2,963,080	3,440,065
Unrealized gains (losses) on available-for-sale securities, net
AOCI [Roll Forward]
Beginning Balance			773,791	1,046,680
OCI before reclassifications			112,423	(137,654)
Amounts reclassified from AOCI			20,959	(4,219)
Other comprehensive income (loss)			133,382	(141,873)
Ending Balance	907,173	904,807	907,173	904,807
Total Accumulated OCI Balance
AOCI [Roll Forward]
Beginning Balance			773,791	1,046,680
OCI before reclassifications			112,423	(137,654)
Amounts reclassified from AOCI			20,959	(4,219)
Other comprehensive income (loss)			133,382	(141,873)
Ending Balance	$ 907,173	$ 904,807	$ 907,173	$ 904,807