Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Outstanding [Line Items]
Short-term borrowings	$ 1,102,140,000	$ 1,033,730,000
Total long-term debt	695,731,000	695,492,000
Short-term FHLB advances
Debt Outstanding [Line Items]
Short-term FHLB advances	$ 700,000,000	700,000,000
Maturity	Apr. 02, 2018
Principal value at period end	$ 700,000,000
Federal Funds Purchased [Member]
Debt Outstanding [Line Items]
Other short-term borrowings	$ 395,000,000	330,000,000
Maturity	Apr. 02, 2018
Principal value at period end	$ 395,000,000
Other short-term borrowings
Debt Outstanding [Line Items]
Other short-term borrowings	7,140,000	3,730,000
Principal value at period end	$ 7,140,000
3.50% Senior Notes
Debt Outstanding [Line Items]
Maturity	Jan. 29, 2025
Principal value at period end	$ 350,000,000
Senior notes	$ 347,386,000	347,303,000
5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity	Sep. 15, 2020
Principal value at period end	$ 350,000,000
Senior notes	$ 348,345,000	$ 348,189,000