Borrowings - Schedule of contractual maturity of borrowings (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
One year or less	$ 761,900
After one year to two years	229,045
After two years to three years	155,010
After three years to four years	43,225
After four years	0
Total FHLB advances	$ 1,189,180	$ 929,057