Income Taxes - Components of Net Deferred Income Taxes (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019
Deferred income tax assets:
Accrued expenses	$ 67,230	$ 59,569
Deferred compensation	36,448	32,765
Stock-based compensation	6,909	6,265
Pension and postretirement benefits	32,437	32,697
Net operating loss carryforwards	5,741	4,570
Deferred rent and tenant allowance	0	24,565
Extended disability benefits	2,779	3,041
Interest rate swaps	14,774	752
State tax credits	19,214	13,420
Operating lease liabilities	86,565	0
Other	1,567	3,804
Total gross deferred income tax assets	273,664	181,448
Less: Valuation allowance	(4,296)	(2,853)
Total net deferred income tax assets	269,368	178,595
Deferred income tax liabilities:
Unbilled receivables	(199,869)	(138,944)
Intangible assets	(64,530)	(60,694)
Debt issuance costs	(1,939)	(3,146)
Property and equipment	(12,820)	(926)
Operating lease right-of-use assets	(62,623)	0
Internally developed software	(15,673)	(8,123)
Total deferred income tax liabilities	(357,454)	(211,833)
Net deferred income tax liabilities	$ (88,086)	$ (33,238)