Investments Unrealized Gains (Losses) on Investments (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Fixed maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Change in net unrealized gains (losses)	$ (6.1)	$ 0.7
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Change in net unrealized gains (losses)		$ 0.9

This item represents the pretax change in net unrealized holding gain (loss) on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

Name:	us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration