Insurance Operations - Retroactive Reinsurance Agreement (Details) - Retroactive reinsurance agreement - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Retention Policy [Line Items]
Reserves	$ 107.8	$ 96.5	$ 306.4
Limit on respective paid losses	40.0	35.0	100.0
Amount in excess of paid losses	119.3	106.5	315.0
Retrospective rating feature, additional premium amount	$ 21.0	$ 18.0	$ 47.6
Retrospective rating feature, additional premium (as a percent)	4.00%	4.00%	3.00%
Reinsurance Recoverable Percentage	100.00%	100.00%	100.00%