Summary of Characteristics of Interest Rate Swaps (Detail) - Interest Rate Swaps - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments:
Current Notional	[1]	$ 30,185,086	$ 31,461,505
Weighted Average Pay Rate	[2],[3]	2.26%	2.49%
Weighted Average Receive Rate	[2]	0.53%	0.22%
Average Years to Maturity	[2]	7 years 7 days	8 years 4 months 17 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]	$ 3,240,436	$ 2,502,505
Weighted Average Pay Rate	[2],[3]	1.85%	1.63%
Weighted Average Receive Rate	[2]	0.36%	0.17%
Average Years to Maturity	[2]	1 year 9 months 18 days	2 years 7 months 21 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]	$ 11,675,000	$ 11,138,000
Weighted Average Pay Rate	[2],[3]	1.82%	2.06%
Weighted Average Receive Rate	[2]	0.55%	0.22%
Average Years to Maturity	[2]	4 years 3 months	5 years 2 months 5 days
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]	$ 11,635,250	$ 13,069,200
Weighted Average Pay Rate	[2],[3]	2.44%	2.67%
Weighted Average Receive Rate	[2]	0.57%	0.23%
Average Years to Maturity	[2]	7 years 11 months 1 day	8 years 6 months 26 days
Greater than 10 years
Derivative Instruments:
Derivative Instruments minimum maturity period		10 years	10 years
Current Notional	[1]	$ 3,634,400	$ 4,751,800
Weighted Average Pay Rate	[2],[3]	3.70%	3.58%
Weighted Average Receive Rate	[2]	0.43%	0.20%
Average Years to Maturity	[2]	19 years 4 months 13 days	19 years 6 months 11 days

[1] Notional amount includes $500.0 million in forward starting pay fixed swaps as of December 31, 2015 and 2014, respectively.

[2] Excludes forward starting swaps.

[3] Weighted average fixed rate on forward starting pay fixed swaps was 1.44% and 3.25% as of December 31, 2015 and 2014, respectively.