Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Fixed interest rate commitments	$ 1,839,190	$ 1,478,157
Variable interest rate commitments	14,821,815	14,034,169
Total loan commitments available for funding	16,661,005	15,512,326
Commercial and standby letters of credit	2,252,016	1,950,211
Total unfunded credit commitments	18,913,021	17,462,537
Commitments unavailable for funding	2,723,835	2,117,057
Allowance for unfunded credit commitments	$ 55,183	$ 51,770	$ 45,265	$ 34,415