Lease Obligations (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets—net	$ 0.0		$ 97.9
Lease liability			$ 100.6
Rent expense	$ 37.8	$ 36.8
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets—net				$ 130.8
Lease liability				132.4
United States Book Business | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets—net				0.6
Lease liability				$ 0.6