Commitments and Contingencies - Commitments and Letters of Credit (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Other Commitments [Line Items]
Commitments and letters of credit	$ 1,533,297	$ 1,501,346
Commitments to extend credit
Other Commitments [Line Items]
Commitments and letters of credit	1,454,312	1,420,428
Standby letters of credit
Other Commitments [Line Items]
Commitments and letters of credit	$ 78,985	$ 80,918