Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) subsidiary segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment
Number of significant subsidiaries | subsidiary	1
Cash on deposit with Federal Reserve Bank	$ 552,000,000	$ 625,300,000
Compensating balances with Federal Reserve Bank	16,100,000	19,300,000
Mortgage loans held for sale	46,635,000	61,794,000
Deferred compensation plan assets	12,200,000	10,600,000
Deferred compensation plan liabilities	12,200,000	10,600,000
Impairment losses	0	200,000	$ 800,000
Write-downs of OREO	(400,000)	(600,000)	(200,000)
Carrying value of foreclosed OREO	2,700,000	2,300,000
Federal Reserve Bank stock	31,800,000	16,400,000
Federal Home Loan Bank stock	10,200,000	10,100,000
Income tax examination, penalties	$ 0	0	0
Number of operating segments | segment	1
Advertising expense	$ 3,500,000	2,800,000	3,500,000
Stock compensation expense	3,900,000	4,400,000	4,000,000
Tax benefits from stock-based compensation	$ 2,600,000	2,100,000	$ 1,400,000
Buildings and improvements | Minimum
Property, Plant and Equipment
Estimated useful lives	5 years
Buildings and improvements | Maximum
Property, Plant and Equipment
Estimated useful lives	45 years
Furniture and equipment | Minimum
Property, Plant and Equipment
Estimated useful lives	4 years
Furniture and equipment | Maximum
Property, Plant and Equipment
Estimated useful lives	15 years
Core Deposits Intangibles
Property, Plant and Equipment
Accumulated amortization	$ 36,900,000	$ 31,500,000