Acquisitions (Details) - Narrative $ / shares in Units, $ in Thousands			3 Months Ended		12 Months Ended
	Jan. 06, 2017 USD ($) $ / shares shares	Mar. 08, 2016 USD ($) location shares	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Feb. 28, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 691,572	$ 339,768			$ 340,879	$ 319,007
Acquisition and integration related expenses			18,565	$ 5,020
Standard Bancshares, Inc.
Business Acquisition [Line Items]
Fixed exchange ratio (in shares) | shares	0.4350
Consideration transferred	$ 580,715
Common stock, shares issued (in shares) | shares	21,057,085
Cash paid	$ 47,125
Goodwill	$ 339,298
Acquisition and integration related expenses			17,500
Standard Bancshares, Inc. | Forecast
Business Acquisition [Line Items]
Acquisition and integration related expenses					$ 27,000
Premier Asset Management LLC
Business Acquisition [Line Items]
Assets under management acquired						$ 550,000
NI Bancshares Corporation
Business Acquisition [Line Items]
Consideration transferred		$ 70,082
Common stock, shares issued (in shares) | shares		3,042,494
Cash paid		$ 15,186
Goodwill		$ 22,174
Number of retail branches acquired | location		10
Minimum value of trust assets under management acquired		$ 700,000
Goodwill adjustment			$ 423
Standard Bancshares, Inc.
Business Acquisition [Line Items]
Share price (in dollars per share) | $ / shares	$ 11.02
First Midwest Bancorp, Inc
Business Acquisition [Line Items]
Share price (in dollars per share) | $ / shares	$ 25.34