Investment Securities - Remaining Contractual Maturity Based on Amortized Cost and Fair Value of HTM by Type (Details) - USD ($) $ in Thousands	Nov. 30, 2019	May 31, 2019
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due in 1 Year or Less	$ 85,967	$ 64,828
Due 1 Year through 5 Years	468,450	475,368
Due 5 Years through 10 Years	15,803	21,795
Due 10 Years	3,327	3,453
Debt securities held-to-maturity, at amortized cost	573,547	565,444
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	86,064	64,722
Due 1 Year through 5 Years	477,704	479,864
Due 5 Years through 10 Years	16,345	22,517
Due 10 Years	3,326	3,446
Debt Securities, Held-to-maturity, Fair Value	$ 583,439	$ 570,549
Due in 1 Year or Less	2.41%	2.08%
Due 1 Year through 5 Years	2.99%	3.10%
Due 5 Years through 10 Years	2.91%	3.07%
Due 10 Years	2.59%	3.26%
Held To Maturity Securities Expected Debt Maturities Weighted Average Yield	2.90%	2.98%
Certificates of deposit
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due in 1 Year or Less	$ 7,687	$ 0
Due 1 Year through 5 Years	1,000	1,000
Due 5 Years through 10 Years	0
Due 10 Years	0
Debt securities held-to-maturity, at amortized cost	8,687	1,000
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	7,688	0
Due 1 Year through 5 Years	1,000	1,000
Due 5 Years through 10 Years	0
Due 10 Years	0
Debt Securities, Held-to-maturity, Fair Value	8,688	1,000
Commercial paper
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due in 1 Year or Less	4,972	12,395
Due 1 Year through 5 Years	0
Due 5 Years through 10 Years	0
Due 10 Years	0
Debt securities held-to-maturity, at amortized cost	4,972	12,395
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	4,972	12,395
Due 1 Year through 5 Years	0
Due 5 Years through 10 Years	0
Due 10 Years	0
Debt Securities, Held-to-maturity, Fair Value	4,972	12,395
U.S. treasury and agency debt securities
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due 1 Year through 5 Years	3,214
Due 5 Years through 10 Years	1,352
Debt securities held-to-maturity, at amortized cost	4,566
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	0
Due 1 Year through 5 Years	3,343
Due 5 Years through 10 Years	1,368
Due 10 Years	0
Debt Securities, Held-to-maturity, Fair Value	4,711
U.S. agency debt securities
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due 1 Year through 5 Years		2,678
Due 5 Years through 10 Years		529
Debt securities held-to-maturity, at amortized cost		3,207
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due 1 Year through 5 Years		2,769
Due 5 Years through 10 Years		546
Debt Securities, Held-to-maturity, Fair Value		3,315
Corporate debt securities
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due in 1 Year or Less	72,443	51,923
Due 1 Year through 5 Years	400,770	414,788
Due 5 Years through 10 Years	10,578	11,867
Due 10 Years	0
Debt securities held-to-maturity, at amortized cost	483,791	478,578
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	72,539	51,818
Due 1 Year through 5 Years	408,850	418,606
Due 5 Years through 10 Years	10,916	12,231
Due 10 Years	0
Debt Securities, Held-to-maturity, Fair Value	492,305	482,655
Agency
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due 1 Year through 5 Years	3,864	310
Due 5 Years through 10 Years	3,371	6,945
Due 10 Years	0
Debt securities held-to-maturity, at amortized cost	7,235	7,255
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	0
Due 1 Year through 5 Years	4,060	317
Due 5 Years through 10 Years	3,559	7,229
Due 10 Years	0
Debt Securities, Held-to-maturity, Fair Value	7,619	7,546
Non-agency
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due 1 Year through 5 Years	0
Due 5 Years through 10 Years	0
Due 10 Years	3,327	3,453
Debt securities held-to-maturity, at amortized cost	3,327	3,453
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	0
Due 1 Year through 5 Years	0
Due 5 Years through 10 Years	0
Due 10 Years	3,326	3,446
Debt Securities, Held-to-maturity, Fair Value	3,326	3,446
Total commercial MBS
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due 1 Year through 5 Years	3,864	310
Due 5 Years through 10 Years	3,371	6,945
Due 10 Years	3,327	3,453
Debt securities held-to-maturity, at amortized cost	10,562	10,708
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	0
Due 1 Year through 5 Years	4,060	317
Due 5 Years through 10 Years	3,559	7,229
Due 10 Years	3,326	3,446
Debt Securities, Held-to-maturity, Fair Value	10,945	10,992
U.S. state and municipality debt securities
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due 1 Year through 5 Years	9,604	9,608
Due 5 Years through 10 Years	0	0
Due 10 Years	0
Debt securities held-to-maturity, at amortized cost	9,604	9,608
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	0
Due 1 Year through 5 Years	10,062	9,960
Due 5 Years through 10 Years	0	0
Due 10 Years	0
Debt Securities, Held-to-maturity, Fair Value	10,062	9,960
Foreign government debt securities
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due 1 Year through 5 Years	1,260	1,254
Due 5 Years through 10 Years	0
Due 10 Years	0
Debt securities held-to-maturity, at amortized cost	1,260	1,254
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	0
Due 1 Year through 5 Years	1,310	1,296
Due 5 Years through 10 Years	0
Due 10 Years	0
Debt Securities, Held-to-maturity, Fair Value	1,310	1,296
Other ABS
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due in 1 Year or Less	865	510
Due 1 Year through 5 Years	48,738	45,730
Due 5 Years through 10 Years	502	2,454
Due 10 Years	0
Debt securities held-to-maturity, at amortized cost	50,105	48,694
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in 1 Year or Less	865	509
Due 1 Year through 5 Years	49,079	45,916
Due 5 Years through 10 Years	502	2,511
Due 10 Years	0
Debt Securities, Held-to-maturity, Fair Value	$ 50,446	$ 48,936