Borrowings - Securities sold under agreements to repurchase and short-term borrowings (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Securities sold under agreements to repurchase
Balance at end of period	$ 240,109	$ 304,566
Weighted average interest rate at end of period (as a percent)	0.69%	0.57%
Maximum outstanding at any month end in year-to-date period	$ 267,596	$ 304,566
Average daily balance for the year-to-date period	$ 246,100	$ 213,527
Weighted average interest rate during period (as a percent)	0.58%	0.46%
Short-term borrowings
Balance at end of period	$ 150,000	$ 220,000
FHLB advances
Short-term borrowings
Balance at end of period	$ 150,000	$ 220,000
Weighted average interest rate at end of period	2.02%	1.42%
Maximum outstanding at any month end in year-to-date period	$ 225,000	$ 234,600
Average daily balance for the year-to-date period	$ 110,083	$ 84,201
Weighted average interest rate during period	1.63%	1.20%