Goodwill, Core Deposit Premium and Other Intangible Assets (Estimated future amortization expense for amortizable intangible assets) (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Future amortization expense for amortizable intangible assets [Line Items]
Nine months ending December 31, 2018	$ 13,233
Year ending December 31,
2019	11,749
2020	10,180
2021	8,640
2022	6,840
2023	5,643
Thereafter	10,249
Total	66,534
Core deposit intangibles
Future amortization expense for amortizable intangible assets [Line Items]
Nine months ending December 31, 2018	5,957
Year ending December 31,
2019	7,092
2020	6,220
2021	5,323
2022	4,175
2023	3,691
Thereafter	8,334
Total	40,792
Customer relationship intangibles
Future amortization expense for amortizable intangible assets [Line Items]
Nine months ending December 31, 2018	29
Year ending December 31,
2019	38
2020	37
2021	36
2022	34
2023	33
Thereafter	64
Total	271
Mortgage servicing rights
Future amortization expense for amortizable intangible assets [Line Items]
Nine months ending December 31, 2018	6,776
Year ending December 31,
2019	4,097
2020	3,512
2021	2,927
2022	2,341
2023	1,756
Thereafter	1,756
Total	23,165
Commercial servicing rights
Future amortization expense for amortizable intangible assets [Line Items]
Nine months ending December 31, 2018	471
Year ending December 31,
2019	522
2020	411
2021	354
2022	290
2023	163
Thereafter	95
Total	$ 2,306