Commitments and Contingencies - Commitments and Letters of Credit (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Other Commitments [Line Items]
Commitments and letters of credit	$ 1,537,680	$ 1,542,026
Commitments to extend credit
Other Commitments [Line Items]
Commitments and letters of credit	1,463,108	1,464,892
Standby letters of credit
Other Commitments [Line Items]
Commitments and letters of credit	$ 74,572	$ 77,134