Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Assets
Securities available for sale, at fair value	$ 4,542,486,000	$ 4,590,062,000
Equity Securities, FV-NI	9,801,000	0
Mortgage loans held for sale	110,348,000	134,916,000
Derivative instruments	11,483,000	29,056,000
Liabilities
Derivative instruments	31,358,000	16,191,000
Fair value assets transferred from Level 1 to Level 2	0
Level 1
Assets
Securities available for sale, at fair value	0	0
Equity Securities, FV-NI	0
Level 2
Assets
Securities available for sale, at fair value	4,542,486,000	4,590,062,000
Equity Securities, FV-NI	9,801,000
Level 3
Assets
Securities available for sale, at fair value	0	0
Equity Securities, FV-NI	0
Recurring
Assets
Securities available for sale, at fair value	4,542,486,000	4,590,062,000
Equity Securities, FV-NI	9,801,000
Mortgage loans held for sale	110,348,000	134,916,000
Derivative instruments	17,072,000	31,265,000
Total, Assets	4,679,707,000	4,756,243,000
Liabilities
Derivative instruments	32,154,000	25,154,000
Total	32,154,000	25,154,000
Recurring | Level 1
Assets
Securities available for sale, at fair value	0	0
Equity Securities, FV-NI	0
Mortgage loans held for sale	0	0
Derivative instruments	0	0
Total, Assets	0	0
Liabilities
Derivative instruments	0	0
Total	0	0
Recurring | Level 2
Assets
Securities available for sale, at fair value	4,542,486,000	4,590,062,000
Equity Securities, FV-NI	9,801,000
Mortgage loans held for sale	110,348,000	134,916,000
Derivative instruments	17,072,000	31,265,000
Total, Assets	4,679,707,000	4,756,243,000
Liabilities
Derivative instruments	32,154,000	25,154,000
Total	32,154,000	25,154,000
Recurring | Level 3
Assets
Securities available for sale, at fair value	0	0
Equity Securities, FV-NI	0
Mortgage loans held for sale	0	0
Derivative instruments	0	0
Total, Assets	0	0
Liabilities
Derivative instruments	0	0
Total	$ 0	$ 0