Premises and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premises and equipment
Total	$ 415,317	$ 296,794
Less accumulated depreciation	(159,752)	(113,284)
Net premises and equipment	255,565	183,510
Depreciation expense charged to operations	15,200	11,500	$ 11,100
Amortization expense	2,500	2,300	$ 1,800
Land
Premises and equipment
Total	81,291	56,454
Buildings and leasehold improvements
Premises and equipment
Total	$ 227,402	157,927
Buildings and leasehold improvements | Minimum
Premises and equipment
Useful Life	15 years
Buildings and leasehold improvements | Maximum
Premises and equipment
Useful Life	40 years
Equipment and furnishings
Premises and equipment
Total	$ 104,319	78,800
Equipment and furnishings | Minimum
Premises and equipment
Useful Life	3 years
Equipment and furnishings | Maximum
Premises and equipment
Useful Life	10 years
Construction in process
Premises and equipment
Total	$ 2,305	3,613
Computer software
Premises and equipment
Useful Life	36 months
Total	$ 13,000	$ 9,700