Investment Securities - Investments Held on Deposit or as Collateral (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Percentage of investments recorded at fair value	73.00%	71.00%
Investments held in trusts	$ 432.4	$ 319.9
Assets held by insurance regulators	$ 11.9	$ 9.9

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=75763970&loc=SL75763979-209797

Name:	us-gaap_AssetsHeldByInsuranceRegulators
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant