Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 segment	Mar. 31, 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 Updated 2017-07 | Other income (expense)
Prior period reclassification adjustment		$ 0.4