Commitments and Contingent Liabilities - Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial instruments with off-balance sheet risk
Contract amount	$ 6,000,000	$ 5,800,000
Commitments to extend credit
Financial instruments with off-balance sheet risk
Contract amount	5,791,088	5,549,591
Standby letters of credit
Financial instruments with off-balance sheet risk
Contract amount	185,872	204,324
Commercial Letters of credit
Financial instruments with off-balance sheet risk
Contract amount	$ 6,994	$ 7,535