Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
	Jan. 29, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Debt Disclosure [Line Items]
Interest expense on debt		$ 9,000,000	$ 5,800,000	$ 25,900,000	$ 17,400,000
Weighted average interest rates associated with short-term borrowings		11.00%		11.00%
Proceeds from issuance of 3.50% Senior Notes				$ 346,431,000	$ 0
6.05% Subordinated Notes
Debt Disclosure [Line Items]
Interest rate on debt		6.05%		6.05%		6.05%
Fair value of the interest rate swap associated with the notes		$ 3,500,000		$ 3,500,000		$ 4,600,000
Principal value at period end		$ 45,964,000		$ 45,964,000
3.50% Senior Notes
Debt Disclosure [Line Items]
Interest rate on debt		3.50%		3.50%
Debt issuance costs		$ 3,000,000		$ 3,000,000
Unamortized discount on debt		300,000		300,000
Principal value at period end	$ 350,000,000	350,000,000		350,000,000
Federal Home Loan Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged		$ 1,300,000,000		$ 1,300,000,000
5.375% Senior Notes
Debt Disclosure [Line Items]
Interest rate on debt		5.375%		5.375%		5.375%
Principal value at period end		$ 350,000,000		$ 350,000,000
Proceeds from issuance of 3.50% Senior Notes	$ 346,400,000
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged		920,000,000		920,000,000
Interest rate swaps | 6.05% Subordinated Notes
Debt Disclosure [Line Items]
Fair value of the interest rate swap associated with the notes		$ 3,500,000		$ 3,500,000		$ 4,600,000