Income Taxes - Components of Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current tax expense (benefit):
Federal									$ 200.9	$ 60.3	$ 73.2
Total current tax expense (benefit)									200.9	60.3	73.2
Deferred tax expense (benefit):
Federal									(9.7)	(65.3)	35.8
Total deferred tax expense (benefit)									(9.7)	(65.3)	35.8
Total income tax expense (benefit)	$ 63.0	$ 33.4	$ 20.6	$ 74.2	$ (21.2)	$ (110.9)	$ 43.1	$ 84.0	$ 191.2	$ (5.0)	$ 109.0