Summary of Characteristics of Interest Rate Swaps (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Derivative Instruments:
Weighted Average Pay Rate	2.49%		2.14%
Weighted Average Receive Rate	0.22%		0.20%
Interest Rate Swaps
Derivative Instruments:
Current Notional	$ 31,461,505	[1]	$ 52,426,910
Weighted Average Pay Rate	2.49%	[2],[3]	2.14%
Weighted Average Receive Rate	0.22%	[2]	0.20%
Average Years to Maturity	8 years 4 months 17 days	[2]	5 years 3 months 4 days
Interest Rate Swaps | 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	2,502,505	[1]	24,286,000
Weighted Average Pay Rate	1.63%	[2],[3]	1.83%
Weighted Average Receive Rate	0.17%	[2]	0.18%
Average Years to Maturity	2 years 7 months 21 days	[2]	1 year 11 months 23 days
Interest Rate Swaps | 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	11,138,000	[1]	8,865,410
Weighted Average Pay Rate	2.06%	[2],[3]	2.02%
Weighted Average Receive Rate	0.22%	[2]	0.19%
Average Years to Maturity	5 years 2 months 5 days	[2]	4 years 2 months 9 days
Interest Rate Swaps | 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	13,069,200	[1]	15,785,500
Weighted Average Pay Rate	2.67%	[2],[3]	2.37%
Weighted Average Receive Rate	0.23%	[2]	0.23%
Average Years to Maturity	8 years 6 months 26 days	[2]	7 years 7 months 28 days
Interest Rate Swaps | Greater than 10 years
Derivative Instruments:
Derivative Instruments minimum maturity period	10 years		10 years
Current Notional	$ 4,751,800	[1]	$ 3,490,000
Weighted Average Pay Rate	3.58%	[2],[3]	3.62%
Weighted Average Receive Rate	0.20%	[2]	0.20%
Average Years to Maturity	19 years 6 months 11 days	[2]	19 years 11 months 5 days

[1] Notional amount includes $500.0 million in forward starting pay fixed swaps.

[2] Excludes forward starting swaps.

[3] Weighted average fixed rate on forward starting pay fixed swaps was 3.25%.