Goodwill, Core Deposit Premium and Other Intangible Assets (Carrying amount of intangible assets (incl accumulated amortization) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 119,897	$ 112,043
Accumulated Amortization	53,363	50,983
Net Carrying Amount	66,534	61,060
Core deposit intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	69,731	62,008
Accumulated Amortization	28,939	27,086
Net Carrying Amount	40,792	34,922
Customer relationship intangibles
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,177	1,177
Accumulated Amortization	906	896
Net Carrying Amount	271	281
Mortgage servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	42,249	42,139
Accumulated Amortization	19,084	18,891
Net Carrying Amount	23,165	23,248
Commercial servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,740	6,719
Accumulated Amortization	4,434	4,110
Net Carrying Amount	$ 2,306	$ 2,609