Accumulated Other Comprehensive Income - Components of AOCI (Detail) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
AOCI [Roll Forward]
Beginning Balance	$ 3,634,977	$ 3,123,533
OCI before reclassifications	(131,157)	37,744
Amounts reclassified from AOCI	5,906	24,433
Net current period OCI	(125,251)	62,177
Ending Balance	3,650,142	3,576,687
Unrealized gains (losses) on available-for-sale securities, net
AOCI [Roll Forward]
Beginning Balance	796,902	718,106
OCI before reclassifications	(131,157)	37,744
Amounts reclassified from AOCI	5,906	24,433
Net current period OCI	(125,251)	62,177
Ending Balance	671,651	780,283
Total Accumulated OCI Balance
AOCI [Roll Forward]
Beginning Balance	796,902	718,106
Ending Balance	$ 671,651	$ 780,283

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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