Summary of Characteristics of Interest Rate Swaps (Detail) - Interest Rate Swaps - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments:
Current Notional	[1]	$ 25,875,665	$ 30,185,086
Weighted Average Pay Rate	[2],[3]	2.22%	2.26%
Weighted Average Receive Rate	[2]	1.02%	0.53%
Weighted Average Years to Maturity	[2]	6 years 10 months 13 days	7 years 7 days
0 - 3 years
Derivative Instruments:
Derivative Instruments minimum maturity period		0 years	0 years
Derivative Instruments maximum maturity period		3 years	3 years
Current Notional	[1]	$ 3,444,365	$ 3,240,436
Weighted Average Pay Rate	[2],[3]	1.37%	1.85%
Weighted Average Receive Rate	[2]	1.00%	0.36%
Weighted Average Years to Maturity	[2]	2 years 8 months 16 days	1 year 9 months 18 days
3 - 6 years
Derivative Instruments:
Derivative Instruments minimum maturity period		3 years	3 years
Derivative Instruments maximum maturity period		6 years	6 years
Current Notional	[1]	$ 10,590,000	$ 11,675,000
Weighted Average Pay Rate	[2],[3]	1.92%	1.82%
Weighted Average Receive Rate	[2]	0.99%	0.55%
Weighted Average Years to Maturity	[2]	3 years 11 months 9 days	4 years 3 months
6 - 10 years
Derivative Instruments:
Derivative Instruments minimum maturity period		6 years	6 years
Derivative Instruments maximum maturity period		10 years	10 years
Current Notional	[1]	$ 8,206,900	$ 11,635,250
Weighted Average Pay Rate	[2],[3]	2.35%	2.44%
Weighted Average Receive Rate	[2]	1.10%	0.57%
Weighted Average Years to Maturity	[2]	7 years 9 months 26 days	7 years 11 months 1 day
Greater than 10 years
Derivative Instruments:
Derivative Instruments minimum maturity period		10 years	10 years
Current Notional	[1]	$ 3,634,400	$ 3,634,400
Weighted Average Pay Rate	[2],[3]	3.70%	3.70%
Weighted Average Receive Rate	[2]	0.83%	0.43%
Weighted Average Years to Maturity	[2]	18 years 4 months 10 days	19 years 4 months 13 days

[1] There were no forward starting swaps as of December 31, 2016. Notional amount includes $500.0 million in forward starting pay fixed swaps as of December 31, 2015.

[2] Excludes forward starting swaps.

[3] There were no forward starting swaps as of December 31, 2016. Weighted average fixed rate on forward starting pay fixed swaps was 1.44% as of December 31, 2015.