Equity - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Mortgage banking and investment activities, net									$ (10,385)	$ 24,792	$ 96,785
Realized gains, net									36,369	15,478	25,259
Net realized loss on interest rate agreements designated as cash flow hedges	$ 25,039	$ 23,875	$ 23,008	$ 23,961	$ 23,903	$ 23,350	$ 21,151	$ 19,059	95,883	87,463	80,971
Net income	$ 41,059	$ 19,164	$ 27,064	$ 14,801	$ 27,122	$ 45,097	$ 16,017	$ 12,333	102,088	100,569	$ 173,246
Other than temporary impairments									419	4,774
Other than temporary impairment losses, income statement									246	565
Accumulated other comprehensive income, other-than-temporary impairments									173	4,209
Net unrealized gains on available-for-sale securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Mortgage banking and investment activities, net									246	565
Realized gains, net									(29,672)	(11,117)
Net income									(29,426)	(10,552)
Net unrealized losses on interest rate agreements accounted for as cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized loss on interest rate agreements designated as cash flow hedges									95	164
Net income									$ (95)	$ (164)