Revenue (Comparison of New and Prior Revenue Recognition Guidance) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Apr. 01, 2018	Mar. 31, 2018
Assets
Accounts receivable, net - current	$ 833.1		$ 833.1		$ 1,042.2	$ 946.0
Other assets - current	227.7		227.7			195.8
Other assets - non-current	453.6		453.6			458.6
Investment in films and television programs and program rights, net	1,670.0		1,670.0			1,692.0
Liabilities
Accounts payable and accrued liabilities	432.5		432.5			447.7
Participations and residuals - current	499.6		499.6			504.5
Participations and residuals - non-current	458.1		458.1			438.3
Deferred revenue - current	173.0		173.0		$ 183.8	183.9
Deferred revenue - non-current	70.0		70.0			70.3
Deferred tax liabilities	46.8		46.8			91.9
Equity
Retained earnings	346.0		346.0			$ 516.6
Statement of Operations Information:
Revenues	933.2	$ 1,142.7	2,766.9	$ 3,088.8
Direct operating	502.0	650.1	1,495.2	1,726.6
Operating income	86.8	80.2	164.0	200.3
Interest and other income	2.9	2.2	9.0	7.7
Loss before income taxes	25.4	(13.1)	(167.1)	173.5
Income tax benefit (provision)	(5.3)	204.2	26.6	205.0
Net income (loss)	20.1	$ 191.1	(140.5)	$ 378.5
Impact of Adoption | Revenue Recognition ASU 2014-09
Assets
Accounts receivable, net - current	(109.2)		(109.2)
Other assets - current	(69.1)		(69.1)
Other assets - non-current	(2.1)		(2.1)
Investment in films and television programs and program rights, net	38.0		38.0
Liabilities
Accounts payable and accrued liabilities	(95.7)		(95.7)
Participations and residuals - current	(26.0)		(26.0)
Deferred revenue - current	(0.5)		(0.5)
Deferred revenue - non-current	0.7		0.7
Deferred tax liabilities	(4.2)		(4.2)
Equity
Retained earnings	(16.7)		(16.7)
Statement of Operations Information:
Revenues	(7.4)		5.7
Direct operating	(2.3)		2.4
Operating income	(5.1)		3.3
Interest and other income	0.1		(0.2)
Loss before income taxes	(5.0)		3.1
Income tax benefit (provision)	1.1		(1.1)
Net income (loss)	(3.9)		2.0
Without Adoption of New Revenue Guidance | Revenue Recognition ASU 2014-09
Assets
Accounts receivable, net - current	723.9		723.9
Other assets - current	158.6		158.6
Other assets - non-current	451.5		451.5
Investment in films and television programs and program rights, net	1,708.0		1,708.0
Liabilities
Accounts payable and accrued liabilities	336.8		336.8
Participations and residuals - current	473.6		473.6
Deferred revenue - current	172.5		172.5
Deferred revenue - non-current	70.7		70.7
Deferred tax liabilities	42.6		42.6
Equity
Retained earnings	329.3		329.3
Statement of Operations Information:
Revenues	925.8		2,772.6
Direct operating	499.7		1,497.6
Operating income	81.7		167.3
Interest and other income	3.0		8.8
Loss before income taxes	20.4		(164.0)
Income tax benefit (provision)	(4.2)		25.5
Net income (loss)	$ 16.2		$ (138.5)