Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jan. 29, 2015	Dec. 31, 2014
Debt Disclosure [Line Items]
Interest expense on debt	$ 9,000	$ 5,800	$ 16,900	$ 11,600
Weighted average interest rates associated with short-term borrowings	0.10%		0.10%
Proceeds from issuance of 3.50% Senior Notes			$ 346,431	$ 0
6.05% Subordinated Notes
Debt Disclosure [Line Items]
Fair value of the interest rate swap associated with the notes	$ 3,800		3,800			$ 4,600
Principal value at period end	45,964		45,964
3.50% Senior Notes
Debt Disclosure [Line Items]
Debt Instrument, Unamortized Discount	300		300
Principal value at period end	350,000		350,000		$ 350,000
Federal Home Loan Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged	1,300,000		1,300,000
5.375% Senior Notes
Debt Disclosure [Line Items]
Principal value at period end	350,000		350,000
Proceeds from issuance of 3.50% Senior Notes	346,400
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged	950,000		950,000
Interest rate swaps | 6.05% Subordinated Notes
Debt Disclosure [Line Items]
Fair value of the interest rate swap associated with the notes	$ 3,800		$ 3,800			$ 4,600