Securities Sold Under Agreements to Repurchase - Remaining Contractual Maturities Of Repurchase Agreements (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	$ 1,867,857	$ 1,864,635
Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,664,427
30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	202,930	110,765
Over 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	500
2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements		1,753,870
U.S. Agencies [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,867,857	1,864,635
U.S. Agencies [Member] | Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	1,664,427
U.S. Agencies [Member] | 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	202,930	110,765
U.S. Agencies [Member] | Over 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements	$ 500
U.S. Agencies [Member] | 2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Total repurchase agreements		$ 1,753,870