Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2018 segment	Jun. 30, 2017 USD ($)	Jan. 01, 2018 USD ($)
Number of reportable segments | segment		1
Interest in subsidiary (less than given percentage)		100.00%
Accounting Standards Update 2016-01
Cumulative effect of new accounting principle				$ 1.5
Accounting Standards Update 2016-01 | Retained earnings
Cumulative effect of new accounting principle				0.0
Accounting Standards Update 2016-16 | Net deferred tax asset
Cumulative effect of new accounting principle				500.0
Accounting Standards Update 2016-16 | Retained earnings
Cumulative effect of new accounting principle				$ 500.0
Accounting Standards Update 2017-07 | Other income (expense)
Prior period reclassification adjustment	$ 0.4		$ 0.8