Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Stranded income tax effects resulting from TCJA reclassified from accumulated other comprehensive income to retained earnings upon adoption of ASU 2018-02	$ (3.1)
Liability for unpaid losses and loss expenses		$ (24.8)	$ (25.5)
Unrealized gains on equity securities reclassified from accumulated other comprehensive income to retained earnings, after tax upon adoption of ASU 2016-01	$ (60.8)