Senior Notes (Details) (Unsecured debt, USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	May 04, 2004	Dec. 13, 2013
7.35% senior notes due 2034
Debt Instrument [Line Items]
Face amount of issued debt			$ 300.0
Stated interest rate (percentage)			7.35%
Estimated fair value of senior notes	462.4	381.0
5.144% senior notes due 2043
Debt Instrument [Line Items]
Face amount of issued debt				500.0
Stated interest rate (percentage)				5.144%
Estimated fair value of senior notes	$ 609.9	$ 514.1