Summary of Characteristics of Interest Rate Swaps (Detail) (Interest Rate Swaps, USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Derivative Instruments:
Current Notional	$ 28,059,988	[1]	$ 31,461,505	[1]
Weighted Average Pay Rate	2.37%	[2],[3]	2.49%	[2],[3]
Weighted Average Receive Rate	0.35%	[2]	0.22%	[2]
Average Years to Maturity	8 years 1 month 2 days	[2]	8 years 4 months 17 days	[2]
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	2,852,488	[1]	2,502,505	[1]
Weighted Average Pay Rate	1.78%	[2],[3]	1.63%	[2],[3]
Weighted Average Receive Rate	0.18%	[2]	0.17%	[2]
Average Years to Maturity	2 years 5 months 12 days	[2]	2 years 7 months 21 days	[2]
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	10,463,000	[1]	11,138,000	[1]
Weighted Average Pay Rate	1.85%	[2],[3]	2.06%	[2],[3]
Weighted Average Receive Rate	0.41%	[2]	0.22%	[2]
Average Years to Maturity	4 years 11 months 27 days	[2]	5 years 2 months 5 days	[2]
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	11,110,100	[1]	13,069,200	[1]
Weighted Average Pay Rate	2.60%	[2],[3]	2.67%	[2],[3]
Weighted Average Receive Rate	0.37%	[2]	0.23%	[2]
Average Years to Maturity	8 years 7 months 21 days	[2]	8 years 6 months 26 days	[2]
Greater than 10 years
Derivative Instruments:
Derivative Instruments minimum maturity period	10 years		10 years
Current Notional	$ 3,634,400	[1]	$ 4,751,800	[1]
Weighted Average Pay Rate	3.70%	[2],[3]	3.58%	[2],[3]
Weighted Average Receive Rate	0.22%	[2]	0.20%	[2]
Average Years to Maturity	20 years 1 month 13 days	[2]	19 years 6 months 11 days	[2]

[1] Notional amount includes $3.0 billion and $500.0 million in forward starting pay fixed swaps as of March 31, 2015 and December 31, 2014, respectively.

[2] Excludes forward starting swaps.

[3] Weighted average fixed rate on forward starting pay fixed swaps was 1.88% and 3.25% as of March 31, 2015 and December 31, 2014, respectively.