Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 3.0	$ 24.7
Net unrealized holding (losses) gains on investments:
Unrealized holding (losses) gains	37.0	17.0
Reclassification adjustments for gains realized in net income	(1.3)	(4.2)
Income tax benefit (expense)	(12.5)	(4.5)
Total net unrealized holding (losses) gains on investments	23.2	8.3
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.4)	(1.4)
Net actuarial loss	2.4	2.9
Income tax expense	(0.4)	(0.5)
Total net unrecognized benefit plan obligations	0.6	1.0
Other comprehensive (loss) income	23.8	9.3
Comprehensive income	$ 26.8	$ 34.0