Business and Significant Accounting Policies - Additional Information (Detail) $ in Millions					12 Months Ended
	Mar. 21, 2018 shares	Feb. 20, 2018 shares	Jun. 15, 2016 shares	May 03, 2016 shares	Dec. 31, 2018 USD ($) segment foundation trust	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)	Mar. 05, 2018 USD ($)	Jan. 01, 2018 USD ($)
Business And Significant Accounting Policies [Line Items]
Number of reportable segments | segment					2
Percentage of contractual participation					100.00%
Number of statutory trusts | trust					12
Number of charitable foundations | foundation					2
Stock dividend declared, percent				10.00%	15.00%		10.00%
Short-term investments, maturity term					1 year
Percentage of investment portfolio in fair value from pricing service or broker (less than)					1.00%
Allowance for doubtful accounts					$ 4.0	$ 3.2
Reinsurance recoverables, allowance for doubtful accounts					1.8	2.1
Estimated liability for return of premiums					6.9	7.9
Estimated amount of premiums receivables due					0.4	0.1
Goodwill					$ 177.0	161.4
Weighted average useful life					9 years 6 months
Amortization expense					$ 3.3	5.9	$ 5.5
Estimated amortization expense for 2019					1.7
Estimated amortization expense for 2020					1.3
Estimated amortization expense for 2021					1.3
Estimated amortization expense for 2022					0.2
Accumulated depreciation for property and equipment					146.0	124.1
Net book value of property and equipment					154.8	151.0
Depreciation expense					24.5	24.1	$ 20.9
Foreign currency translation adjustment					$ 22.4	19.0
US federal tax rate					21.00%
Accounting Standards Update 2018-02 [Member] | Retained Earnings [Member]
Business And Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASU, net of taxes						(20.7)
Accounting Standards Update 2018-02 [Member] | AOCI Attributable to Parent [Member]
Business And Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASU, net of taxes						20.7				$ 20.7
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Business And Significant Accounting Policies [Line Items]
Right-of-use asset								$ 118.0
Operating lease, liability								$ 145.0
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Business And Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASU, net of taxes						117.5
Accounting Standards Update 2016-01 [Member] | AOCI Attributable to Parent [Member]
Business And Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASU, net of taxes						$ (117.5)				$ (117.5)
Minimum [Member]
Business And Significant Accounting Policies [Line Items]
Useful life					3 years
Minimum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Business And Significant Accounting Policies [Line Items]
Lessee, operating lease, discount rate								1.19%
Maximum [Member]
Business And Significant Accounting Policies [Line Items]
Useful life					39 years
Maximum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Business And Significant Accounting Policies [Line Items]
Lessee, operating lease, discount rate								5.07%
Distribution Network [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life					9 years 8 months 12 days
Other [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life					6 years 3 months 18 days
Stock Dividend [Member]
Business And Significant Accounting Policies [Line Items]
Stock dividend declared, percent		15.00%		10.00%
Additional stock issued as dividend (in shares) | shares	4,397,520	4,397,520	2,735,542	2,735,542
Ariscom Compagnia Di Assicurazioni S P A [Member]
Business And Significant Accounting Policies [Line Items]
Percentage of voting interests acquired									100.00%
Goodwill					$ 15.6				$ 15.6