Short-Term Borrowings and Long-Term Debt - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended								12 Months Ended
	Jan. 29, 2015	Sep. 30, 2010	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	May 15, 2007	Oct. 30, 2003
Debt Disclosure [Line Items]
Interest expense, debt											$ 37,300,000	$ 34,900,000	$ 23,200,000
Weighted average interest rates associated with short-term borrowings			0.59%				0.32%				0.59%	0.32%
Short-term borrowings			$ 512,668,000				$ 774,900,000				$ 512,668,000	$ 774,900,000
Proceeds from issuance of senior notes, net of discount and issuance cost											0	346,431,000	0
Interest expense			$ 10,728,000	$ 11,252,000	$ 10,656,000	$ 10,237,000	$ 10,163,000	$ 10,131,000	$ 10,155,000	$ 9,891,000	42,873,000	$ 40,340,000	$ 35,321,000
Special Purpose Trust Entity
Debt Disclosure [Line Items]
Interest expense											$ 3,500,000
3.50% Senior Notes
Debt Disclosure [Line Items]
Stated interest rate			3.50%				3.50%				3.50%	3.50%
Principal value	$ 350,000,000		$ 350,000,000								$ 350,000,000
Debt Instrument, Unamortized Discount			$ 300,000								$ 300,000
Maturity											Jan. 29, 2025
5.375% Senior Notes
Debt Disclosure [Line Items]
Stated interest rate			5.375%				5.375%				5.375%	5.375%
Principal value		$ 350,000,000	$ 350,000,000								$ 350,000,000
Maturity											Sep. 15, 2020
Payments for settlement of 3.875% Convertible Notes		250,000,000
Proceeds from issuance of senior notes, net of discount and issuance cost	$ 346,400,000	$ 345,000,000
6.05% Subordinated Notes
Debt Disclosure [Line Items]
Debt Instrument, Unamortized Discount														$ 250,000,000
7.0% Junior Subordinated Debentures
Debt Disclosure [Line Items]
Principal value															$ 50,000,000.0
Debt issuance costs															$ 2,200,000
Federal Funds Purchased
Debt Disclosure [Line Items]
Principal value			0								$ 0
Short-term borrowings			0				$ 135,000,000				0	$ 135,000,000
Federal Home Loan Bank Advances
Debt Disclosure [Line Items]
Principal value			500,000,000								$ 500,000,000
Maturity											Jan. 06, 2017
Market value of collateral pledged			1,700,000,000								$ 1,700,000,000
Unused and available funds			1,200,000,000								1,200,000,000
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Market value of collateral pledged			$ 800,000,000								$ 800,000,000
6.05% Subordinated Notes
Debt Disclosure [Line Items]
Stated interest rate			6.05%				6.05%				6.05%	6.05%
Principal value			$ 45,964,000								$ 45,964,000
Maturity											Jun. 01, 2017
7.0% Junior Subordinated Debentures
Debt Disclosure [Line Items]
Stated interest rate			7.00%				7.00%				7.00%	7.00%
Principal value			$ 50,000,000								$ 50,000,000
Maturity											Oct. 15, 2033