Variable Interest Entities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Investment securities related to state and political subdivisions	$ 1,250,000	$ 1,350,000
Variable interest entity, other short-term borrowings	$ 1,080,000	$ 1,160,000
Weighted average life of trusts	4 years 7 months	4 years 6 months
Total standby bond-purchase agreement committed to trusts	$ 1,100,000
Total letters of credit committed to trusts	351,000
Standby purchase agreements and letters of credit commitments utilized	0
VIE - primary beneficiary
Variable Interest Entity [Line Items]
Assets	149,390	$ 0
Liabilities	49,780	0
Potential maximum loss exposure of unconsolidated funds	100,000
VIE - not primary beneficiary
Variable Interest Entity [Line Items]
Potential maximum loss exposure of unconsolidated funds	$ 72,000	$ 121,000