Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Debt Outstanding [Line Items]
Short-term borrowings	$ 2,421,000	$ 774,900,000
Total long-term debt	795,971,000	796,702,000
Short-term FHLB advances
Debt Outstanding [Line Items]
Short-term FHLB advances	$ 0	638,000,000
Maturity
Principal value at period end	$ 0
Federal funds purchased
Debt Outstanding [Line Items]
Other short-term borrowings	0	135,000,000
Principal value at period end	0
Other short-term borrowings
Debt Outstanding [Line Items]
Other short-term borrowings	2,421,000	1,900,000
Principal value at period end	$ 2,421,000
3.50% Senior Notes
Debt Outstanding [Line Items]
Maturity	Jan. 29, 2025
Principal value at period end	$ 350,000,000
Senior notes	$ 346,900,000	346,667,000
5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity	Sep. 15, 2020
Principal value at period end	$ 350,000,000
Senior notes	$ 347,440,000	347,016,000
6.05% Subordinated Notes
Debt Outstanding [Line Items]
Maturity	Jun. 01, 2017
Principal value at period end	$ 45,964,000
Subordinated notes	$ 47,094,000	48,350,000
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Maturity	Oct. 15, 2033
Principal value at period end	$ 50,000,000
Junior subordinated debentures	$ 54,537,000	$ 54,669,000