Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Debt Disclosure [Line Items]
Interest expense on debt	$ 9,000	$ 8,000
Federal Home Loan Bank, Advances, General Debt Obligations, Amount of Available, Unused Funds	1,790,000
3.50% Senior Notes
Debt Disclosure [Line Items]
Principal value at period end	$ 350,000
Interest rate on debt	3.50%		3.50%
5.375% Senior Notes
Debt Disclosure [Line Items]
Principal value at period end	$ 350,000
Interest rate on debt	5.375%		5.375%
6.05% Subordinated Notes
Debt Disclosure [Line Items]
Principal value at period end	$ 45,964
Interest rate on debt	6.05%		6.05%
Fair value of the interest rate swap associated with the notes	$ 2,400		$ 2,800
Short-term FHLB advances
Debt Disclosure [Line Items]
Market value of collateral pledged	1,800,000
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged	900,000
Amount of Available Unused Funds	878,000
Interest rate swaps | 6.05% Subordinated Notes
Debt Disclosure [Line Items]
Fair value of the interest rate swap associated with the notes	$ 2,400		$ 2,800