Securities Sold Under Agreements to Repurchase - Remaining Contractual Maturities Of Repurchase Agreements (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	$ 1,929,107	$ 1,864,635
Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,563,679
2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	147,842	1,753,870
30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	217,086	110,765
Over 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	500
U.S. Agencies [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,929,107	1,864,635
U.S. Agencies [Member] | Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,563,679
U.S. Agencies [Member] | 2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	147,842	1,753,870
U.S. Agencies [Member] | 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	217,086	$ 110,765
U.S. Agencies [Member] | Over 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	$ 500