Commitments and Contingent Liabilities - Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial instruments with off-balance sheet risk
Contract amount	$ 5,700,000	$ 5,800,000
Commitments to extend credit
Financial instruments with off-balance sheet risk
Contract amount	5,468,976	5,549,591
Standby letters of credit
Financial instruments with off-balance sheet risk
Contract amount	186,321	204,324
Commercial Letters of credit
Financial instruments with off-balance sheet risk
Contract amount	$ 9,602	$ 7,535