Description of Business, Basis of Presentation, and Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Description of Business, Basis of Presentation, and Summary of Significant Accounting Policies [Line Items]
Depreciation expense	$ 10.1	$ 11.2	$ 7.6
Guarantee maturity value as percentage of net premiums	75.00%
Accounting Standards Update 2016-09 [Member]
Description of Business, Basis of Presentation, and Summary of Significant Accounting Policies [Line Items]
Reduction of income tax expense for excess tax benefit of share-based transactions	$ 6.1
Minimum [Member]
Description of Business, Basis of Presentation, and Summary of Significant Accounting Policies [Line Items]
Interest rate reserve assumptions	3.50%	3.50%
Maximum [Member]
Description of Business, Basis of Presentation, and Summary of Significant Accounting Policies [Line Items]
Interest rate reserve assumptions	7.00%	7.00%
Noncompete Agreements [Member]
Description of Business, Basis of Presentation, and Summary of Significant Accounting Policies [Line Items]
Intangible asset amortization expense	$ 3.4	$ 3.4	$ 3.4
Intangible assets, remaining amortization expense, 2018	3.4
Intangible assets, remaining amortization expense, 2019	$ 2.8
Intangible assets, life	24 years