Derivative Instruments - Narrative (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative assets	$ 2,832,000		$ 2,832,000		$ 15,460,000
Realized gains (losses) on terminations of interest rate swaps	$ 60,158,000	$ 31,124,000	$ 60,616,000	$ 99,703,000
The weighted average pay rate on the Company's interest rate swaps	2.07%		2.07%		1.51%
The weighted average receive rate on the Company's interest rate swaps	0.62%		0.62%		0.36%
The weighted average maturity on the Company's interest rate swaps			8 years		4 years
Fair value of credit risk derivative instruments in a net liability position	$ 108,000,000		$ 108,000,000
Early Terminated Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative instruments terminated	$ 2,265,000,000	$ 600,000,000	$ 3,265,000,000	$ 1,200,000,000.0