Summary of Significant Accounting Policies Losses, Claims and Settlement Expenses - General (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information, by Segment
Workers' Compensation Discount, Amount	$ 209,600,000	$ 216,500,000	$ 240,700,000
Interest Accretion Amount	34,500,000	49,000,000.0	$ 20,400,000
General liability insurance face amount	10,000,000.0
Retention payable	500,000
Losses, claims, and settlement expenses, gross	126,800,000	105,800,000
Losses, claims, and settlement expenses	$ 83,300,000	$ 74,400,000
Average five year survival ratio, gross	6 years 3 months 18 days	4 years 3 months 18 days
Average five year survival ratio, net	7 years 2 months 12 days	5 years
Minimum [Member]
Supplementary Insurance Information, by Segment
Workers' compensation discount, percent	3.50%
General liability insurance face amount	$ 1,000,000.0
Maximum [Member]
Supplementary Insurance Information, by Segment
Workers' compensation discount, percent	4.00%
General liability insurance face amount	$ 2,000,000.0