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]	$ 29,877,369	$ 30,185,086
Weighted Average Pay Rate	[2],[3]	2.26%	2.26%
Weighted Average Receive Rate	[2]	0.69%	0.53%
Average Years to Maturity	[2]	6 years 9 months 4 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]	$ 4,290,419	$ 3,240,436
Weighted Average Pay Rate	[2],[3]	1.79%	1.85%
Weighted Average Receive Rate	[2]	0.47%	0.36%
Average Years to Maturity	[2]	1 year 10 months 13 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]	$ 11,925,000	$ 11,675,000
Weighted Average Pay Rate	[2],[3]	1.87%	1.82%
Weighted Average Receive Rate	[2]	0.73%	0.55%
Average Years to Maturity	[2]	4 years 2 months 19 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]	$ 10,227,550	$ 11,635,250
Weighted Average Pay Rate	[2],[3]	2.49%	2.44%
Weighted Average Receive Rate	[2]	0.76%	0.57%
Average Years to Maturity	[2]	7 years 10 months 17 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]	$ 3,434,400	$ 3,634,400
Weighted Average Pay Rate	[2],[3]	3.54%	3.70%
Weighted Average Receive Rate	[2]	0.59%	0.43%
Average Years to Maturity	[2]	18 years 7 months 21 days	19 years 4 months 13 days

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