Derivative Instruments - Cash Flow Hedges (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Rate Cash Flow Hedges
Issuance of long-term debt	$ 435.0	$ 400.0	$ 2,150.0
Proceeds received upon settlement recorded in OCI			19.0
Reclassified from accumulated OCI into income	2.2	$ 2.2	1.2
Reclassification within next 12 months	$ 2.2
Total senior notes issued in June 2015
Interest Rate Cash Flow Hedges
Issuance of long-term debt			$ 1,200.0