Goodwill and Core Deposit Intangibles - Goodwill Narrative (Details) - USD ($)				12 Months Ended
	Jul. 01, 2017	Jul. 01, 2016	Jul. 01, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Change in goodwill				$ 231,900,000	$ 8,300,000
Percent change in goodwill				109.00%
Goodwill				$ 444,704,000	$ 212,820,000	$ 204,500,000	$ 205,600,000
Impairment of goodwill	$ 0	$ 0	$ 0
MWFC Acquisition
Goodwill [Line Items]
Change in goodwill						$ (1,100,000)