Investment in CyrusOne - Schedule of Related Party Transactions (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Amounts receivable from and payable to CyrusOne
Dividends receivable	$ 1.2	$ 2.7	$ 1.2	$ 2.7
CyrusOne [Member]
Revenue:
Services provided to CyrusOne	0.3	0.3	0.9	1.0
Operating costs and expenses:
Charges for services provided by CyrusOne	2.4	2.6	7.6	7.6
Administrative services provided to CyrusOne	(0.1)	(0.1)	(0.2)	(0.3)
Total operating costs and expenses	2.3	$ 2.5	7.4	$ 7.3
Amounts receivable from and payable to CyrusOne
Accounts receivable	0.0		0.0		$ 0.1
Dividends receivable	1.2		1.2		2.1
Receivable from CyrusOne	1.2		1.2		2.2
Payable to CyrusOne	$ 1.4		$ 1.4		$ 1.5