Commitments and Contingent Liabilities - Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Commitments to extend credit
Financial instruments with off-balance sheet risk
Contract amount	$ 5,419,907	$ 5,401,763
Standby letters of credit
Financial instruments with off-balance sheet risk
Contract amount	162,663	161,798
Commercial Letters of credit
Financial instruments with off-balance sheet risk
Contract amount	$ 6,076	$ 5,540