Other Comprehensive Income (Reconciliation of Gross Unrealized Investment Gains) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Other Comprehensive Income
Unrealized holding gains arising during period, net of income tax benefit (expense)	$ 52.7	$ 57.2	$ 64.3	$ 103.3
Less: reclassification adjustment for gains (losses) included in net income, net of income tax (expense) benefit	15.6	0.4	20.6	9.1
Total available-for-sale securities	37.1	56.8	43.7	94.2
Unrealized losses arising during period, net of income tax benefit of $0.7 for the quarter and six months ended June 30, 2011	(1.2)		(1.2)
Unrealized holding gains arising during period, tax expense	(0.1)	(11.0)	5.1	(31.7)
Reclassification adjustment for gains (losses) included in net income, tax (expense) benefit	2.2	0.2	4.3	(0.2)
Benefit for deferred federal income taxes	0.7		0.7
Other comprehensive income	$ 35.9	$ 56.8	$ 42.5	$ 94.2