Commitments and Contingencies - Commitments and Letters of Credit (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Commitments and letters of credit	$ 1,507,295	$ 1,594,230
Commitments to Extend Credit
Other Commitments [Line Items]
Commitments and letters of credit	1,421,906	1,509,696
Standby Letters of Credit
Other Commitments [Line Items]
Commitments and letters of credit	$ 85,389	$ 84,534