Commitments and Contingent Liabilities - Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial instruments with off-balance sheet risk
Contract amount	$ 5,600,000	$ 5,200,000
Commitments to extend credit
Financial instruments with off-balance sheet risk
Contract amount	5,401,763	5,121,811
Standby letters of credit
Financial instruments with off-balance sheet risk
Contract amount	161,798	60,848
Commercial Letters of credit
Financial instruments with off-balance sheet risk
Contract amount	$ 5,540	$ 6,813