Investment in CyrusOne - Schedule of Related Party Transactions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Amounts receivable from and payable to CyrusOne
Dividends receivable	$ 4.5	$ 6.0	$ 4.5	$ 6.0
Receivable from CyrusOne	5.2		5.2		$ 7.7
Payable to CyrusOne	1.6		1.6		0.4
CyrusOne [Member]
Related Party Transaction [Line Items]
Income Tax Receivable from Related Party	0.7		0.7		1.7
Revenue:
Services provided to CyrusOne	0.4	0.4	0.7	0.9
Operating costs and expenses:
Charges for services provided by CyrusOne	2.5	2.3	5.0	4.6
Administrative services provided to CyrusOne	(0.1)	(0.1)	(0.2)	(0.2)
Total operating costs and expenses	2.4	$ 2.2	4.8	$ 4.4
Amounts receivable from and payable to CyrusOne
Accounts receivable	0.7		0.7		1.7
Dividends receivable	4.5		4.5		6.0
Receivable from CyrusOne	5.2		5.2		7.7
Accounts payable	1.6		1.6		0.4
Payable to CyrusOne	$ 1.6		$ 1.6		$ 0.4