Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Accounting Policies [Abstract]
Number of reportable segments | segment	4
Insurance policy duration (years)	1 year
Recorded allowances for credit losses (less than)	$ 1,500,000	$ 1,500,000
Estimated credit losses (more than)	500,000	500,000
Earned but unbilled premiums	$ 4,300,000	4,300,000
Minimum period for claims resolution (years)	5 years
Property, Plant and Equipment [Line Items]
Estimated aggregate amortization of intangible assets for 2018	$ 6,200,000
Estimated aggregate amortization of intangible assets for 2019	6,100,000
Estimated aggregate amortization of intangible assets for 2020	6,100,000
Estimated aggregate amortization of intangible assets for 2021	6,000,000
Estimated aggregate amortization of intangible assets for 2022	$ 6,000,000
Number of reporting units | segment	2
Goodwill impairment loss	$ 0	0	$ 0
Building and Building Improvements
Property, Plant and Equipment [Line Items]
Real estate accumulated depreciation	24,000,000	22,900,000
Real estate depreciation expense	$ 1,100,000	$ 1,400,000	$ 1,500,000