Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Liabilities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Level 3 Financial Liabilities:
Balance, beginning of period	$ 93.7		$ 81.8		$ 74.8		$ 79.5
Net purchases/(sales)	9.4		1.0		16.6		2.7
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	0		0		0		0
Realized (gains) losses	0.4	[1]	(5.9)	[1]	12.1	[1]	(5.3)	[1]
Balance, end of period	$ 103.5		$ 76.9		$ 103.5		$ 76.9

[1] Realized gains and losses are included in net realized investment gains on the statements of income and comprehensive income.