Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Fixed interest rate commitments	$ 2,434,042	$ 1,839,190
Variable interest rate commitments	19,309,317	14,821,815
Total loan commitments available for funding	21,743,359	16,661,005
Commercial and standby letters of credit	2,778,561	2,252,016
Total unfunded credit commitments	24,521,920	18,913,021
Commitments unavailable for funding	3,051,075	2,723,835
Allowance for unfunded credit commitments	$ 67,656	$ 55,183	$ 51,770	$ 45,265