Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Outstanding [Line Items]
Short-term borrowings	$ 417,246,000	$ 1,033,730,000
Long-term debt	$ 695,972,000	695,492,000
Senior Notes | 3.50% Senior Notes
Debt Outstanding [Line Items]
Maturity	Jan. 29, 2025
Principal value at period end	$ 350,000,000
Long-term debt	$ 347,470,000	$ 347,303,000
Interest rate on senior notes	3.50%	3.50%
Senior Notes | 5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity	Sep. 15, 2020
Principal value at period end	$ 350,000,000
Long-term debt	$ 348,502,000	$ 348,189,000
Interest rate on senior notes	5.375%	5.375%
Short-term FHLB advances
Debt Outstanding [Line Items]
Maturity	Jul. 02, 2018
Principal value at period end	$ 400,000,000
Short-term borrowings	$ 400,000,000	$ 700,000,000
Federal funds purchased
Debt Outstanding [Line Items]
Maturity
Principal value at period end	$ 0
Short-term borrowings	0	330,000,000
Short-term borrowings
Debt Outstanding [Line Items]
Principal value at period end	17,246,000
Short-term borrowings	$ 17,246,000	$ 3,730,000