Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Excluding Letters of Credit) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Disclosure Off Balance Sheet Arrangements Guarantees And Other Commitments Additional Information [Abstract]
Fixed interest rate commitments	$ 1,848,611	$ 1,839,190
Variable interest rate commitments	16,153,959	14,821,815
Total loan commitments available for funding	18,002,570	16,661,005
Commercial and standby letters of credit	2,264,959	2,252,016
Total unfunded credit commitments	20,267,529	18,913,021
Commitments unavailable for funding	2,746,202	2,723,835
Reserve for unfunded commitments	$ 57,970	$ 55,183	$ 52,823	$ 51,770