Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (24.5)	$ 33.4	$ 6.0	$ (2.1)	$ (13.8)	$ (9.5)	$ 8.7	$ (3.3)	$ 12.8	$ (17.8)	$ 19.2
Net change in unrealized holding gains (losses) on investments:
Unrealized holding gains (losses) arising during year									(46.4)	69.0	27.8
Reclassification adjustments for gains realized in net income									(2.0)	(65.1)	(36.5)
Income tax (expense) benefit									10.1	(0.7)	3.0
Total change in net unrealized holding gains (losses) on investments									(38.3)	3.2	(5.7)
Net unrecognized benefit plan obligations:
Net actuarial (loss) gain arising during period									(4.3)	(1.5)	(4.6)
Reclassification adjustments for amortization to statements of income:
Negative prior service cost									(6.3)	(6.3)	(6.2)
Net actuarial gain									13.2	12.5	14.9
Income tax expense									(0.6)	(1.7)	(1.5)
Total net unrecognized benefit plan obligations									2.0	3.0	2.6
Other comprehensive income (loss)									(36.3)	6.2	(3.1)
Comprehensive (loss) income									$ (23.5)	$ (11.6)	$ 16.1