Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Excluding Letters of Credit) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Disclosure Off Balance Sheet Arrangements Guarantees And Other Commitments Additional Information [Abstract]
Fixed interest rate commitments	$ 1,462,728		$ 1,475,179
Variable interest rate commitments	13,037,817		13,572,161
Total loan commitments available for funding	14,500,545		15,047,340
Commercial and standby letters of credit	1,841,385		1,695,856
Total unfunded credit commitments	16,341,930		16,743,196
Commitments unavailable for funding	2,209,680		1,719,524
Reserve For Unfunded Credit Commitments	$ 48,172	$ 47,000	$ 45,265	$ 35,924	$ 34,889	$ 34,415