Summary of Long-Term Debt Outstanding (Detail 2) - USD ($)		12 Months Ended
	Nov. 06, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Increases in the carrying values of notes outstanding covered by interest-rate swap contracts			$ 0	$ (900,000)
Senior Notes [Member]
Debt Instrument [Line Items]
Offering Circular, ability to offer notes in aggregate principal amount	$ 4,500,000,000.0
Senior Notes [Member] | Upper Limit
Debt Instrument [Line Items]
Debt maturity	15 years
Senior Notes [Member] | Lower Limit
Debt Instrument [Line Items]
Debt maturity	30 days
Notes 6.50 Percent Bank Subordinated Debt Due August 2018
Debt Instrument [Line Items]
Interest Rate		6.50%	6.50%
Notes issued at a discount		0.02%	0.02%
Notes 6.50 Percent Bank Subordinated Debt Due August 2018 | Upper Limit
Debt Instrument [Line Items]
Debt maturity	15 years
Notes 6.50 Percent Bank Subordinated Debt Due August 2018 | Lower Limit
Debt Instrument [Line Items]
Debt maturity	5 years
Subordinated Debt [Member]
Debt Instrument [Line Items]
Offering Circular, ability to offer notes in aggregate principal amount	$ 1,000,000,000.0
Subordinated Debt [Member] | Interest Rate Swaps
Debt Instrument [Line Items]
Increases in the carrying values of notes outstanding covered by interest-rate swap contracts		$ 92,400,000	$ 100,600,000
Fixed Rate Corporate Senior Notes 3.45 Percent Due November 2020 [Member]
Debt Instrument [Line Items]
Interest Rate		3.45%
Notes issued at a discount		0.117%	0.117%
Fixed Rate Corporate Senior Notes 3.38 Percent Due August 2021 [Member]
Debt Instrument [Line Items]
Interest Rate		3.38%
Notes issued at a discount		0.437%	0.437%
Fixed Rate Corporate Senior Notes Two Point Three Eight Percent Due August Twenty Twenty Two [Member]
Debt Instrument [Line Items]
Interest Rate		2.38%
Notes issued at a discount		0.283%	0.283%