Borrowed Funds - Carrying Amounts and Market Values of Securities and Related Repurchase Liabilities and Weighted Average Interest Rates of Repurchase Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Securities Fair Market Value	$ 2,130,670	$ 1,887,010
Repurchase Liabilities	$ 2,249,861	$ 1,864,635
Weighted Average Interest Rate	0.52%	1.52%
2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Fair Market Value	$ 1,625,230	$ 1,772,820
Repurchase Liabilities	$ 1,759,559	$ 1,753,870
Weighted Average Interest Rate	0.34%	1.44%
30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Fair Market Value	$ 504,435	$ 114,190
Repurchase Liabilities	$ 489,302	$ 110,765
Weighted Average Interest Rate	1.17%	2.72%
Over 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Fair Market Value	$ 1,005
Repurchase Liabilities	$ 1,000