Reinsurance - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Mar. 01, 2018	Dec. 31, 2017
Reinsurance [Line Items]
Ceded	$ (465.2)	$ 9.7	$ (417.5)
Document Period End Date	Dec. 31, 2018
Unearned premium	$ 584.2		$ 611.8

Carrying amount of premiums written on insurance contracts that have not been earned as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(13)(a)(2))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Name:	us-gaap_UnearnedPremiums
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant