Derivative Financial Instruments - Information about Fair Values of Derivative Instruments in Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 31, 2020
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	$ 703,786	$ 1,067,985
Liability Derivatives, Fair Value	113,520	140,485
Derivatives Not Designated and Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	697,344	1,064,529
Liability Derivatives, Fair Value	110,008	131,135
Fair Value Hedges [Member] | Derivatives Designated and Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	6,442	3,456
Liability Derivatives, Fair Value	3,512	9,350
Interest Rate Swap Agreements [Member] | Fair Value Hedges [Member] | Derivatives Designated and Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	172	1,968
Liability Derivatives, Fair Value	3,366	892
Commitments to Sell Real Estate Loans [Member] | Derivatives Not Designated and Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	36,179	2,409
Liability Derivatives, Fair Value	5,605	13,868
Commitments to Sell Real Estate Loans [Member] | Fair Value Hedges [Member] | Derivatives Designated and Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	6,270	1,488
Liability Derivatives, Fair Value	146	8,458
Mortgage-Related Commitments to Originate Real Estate Loans for Sale [Member] | Derivatives Not Designated and Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	22,921	43,599
Liability Derivatives, Fair Value	9,164	365
Interest Rate Contracts [Member] | Derivatives Not Designated and Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	629,458	1,008,913
Liability Derivatives, Fair Value	84,567	105,768
Foreign Exchange and Other Option and Futures Contracts [Member] | Derivatives Not Designated and Qualifying as Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair Value	8,786	9,608
Liability Derivatives, Fair Value	$ 10,672	$ 11,134