Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) Payment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Home office and equipment [Abstract]
Accumulated depreciation of home office and equipment	$ 38,100,000	$ 33,900,000	$ 30,600,000
Depreciation expense of home office and equipment	$ 6,000,000	5,400,000	4,600,000
Minimum number of payments past due to be in default | Payment	2
Maximum age for qualified employees for both medical and dental benefits	65 years
Related party transaction amount	$ 0	$ 0	$ 0
Minimum
Home office and equipment [Abstract]
Estimated useful life	3 years
Award vesting period	1 year
Maximum
Home office and equipment [Abstract]
Estimated useful life	45 years
Award vesting period	3 years