Consolidated Statements of Stockholders Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income, Net	Treasury Stock at Cost
Stockholders' equity, shares at Dec. 31, 2018		56,975,675
Stockholders' equity, value at Dec. 31, 2018	$ 1,018.2	$ 0.6	$ 388.8	$ 1,030.7	$ (13.7)	$ (388.2)
Stock-based obligations (Note 14), shares		0
Stock-based obligations (Note 14), value	10.1	$ 0.0	10.1	0.0	0.0	0.0
Stock-options exercised, shares		31,630
Stock options exercised, value	0.7	$ 0.0	0.7	0.0	0.0	0.0
Vesting of restricted stock units, net of shares withheld to satisfy minimum tax withholding (Note 14), shares		177,065
Vesting of restricted stock units, net of shares withheld to satisfy minimum tax withholding (Note 14), value	(3.2)	$ 0.0	(3.2)	0.0	0.0	0.0
Acquisition of treasury stock (Note 13), shares		0
Acquisition of treasury stock (Note 13), value	(67.1)	$ 0.0	0.0	0.0	0.0	(67.1)
Dividend to common stockholders, shares		0
Dividend to common stockholders, value	28.9	$ 0.0	0.0	28.9	0.0	0.0
Net income for the period	157.1	0.0	0.0	157.1	0.0	0.0
Net impact of tax enactment on net unrealized gains on investments	0.0	0.0	0.0	0.0	0.0	0.0
Change in net unrealized gains on investments, net of taxes	79.0	$ 0.0	0.0	0.0	79.0	0.0
Stockholders' equity, shares at Dec. 31, 2019		57,184,370
Stockholders' equity, value at Dec. 31, 2019	1,165.8	$ 0.6	396.4	1,158.8	65.3	(455.3)
Stock-based obligations (Note 14), shares		0
Stock-based obligations (Note 14), value	9.7	$ 0.0	9.7	0.0	0.0	0.0
Stock-options exercised, shares		40,800
Stock options exercised, value	0.9	$ 0.0	0.9	0.0	0.0	0.0
Vesting of restricted stock units, net of shares withheld to satisfy minimum tax withholding (Note 14), shares		188,636
Vesting of restricted stock units, net of shares withheld to satisfy minimum tax withholding (Note 14), value	(2.7)	$ 0.0	(2.7)	0.0	0.0	0.0
Acquisition of treasury stock (Note 13), shares		0
Acquisition of treasury stock (Note 13), value	(99.8)	$ 0.0	0.0	0.0	0.0	(99.8)
Dividend to common stockholders, shares		0
Dividend to common stockholders, value	30.8	$ 0.0	0.0	30.8	0.0	0.0
Net income for the period	119.8	0.0	0.0	119.8	0.0	0.0
Change in net unrealized gains on investments, net of taxes	$ 49.8	$ 0.0	0.0	0.0	49.8	0.0
Stockholders' equity, shares at Dec. 31, 2020	57,413,806	57,413,806
Stockholders' equity, value at Dec. 31, 2020	$ 1,212.8	$ 0.6	404.3	1,247.9	115.1	(555.1)
Stock-based obligations (Note 14), shares		0
Stock-based obligations (Note 14), value	9.1	$ 0.0	9.1	0.0	0.0	0.0
Stock-options exercised, shares		48,051
Stock options exercised, value	1.1	$ 0.0	1.1	0.0	0.0	0.0
Vesting of restricted stock units, net of shares withheld to satisfy minimum tax withholding (Note 14), shares		228,397
Vesting of restricted stock units, net of shares withheld to satisfy minimum tax withholding (Note 14), value	(3.8)	$ 0.0	(3.8)	0.0	0.0	0.0
Acquisition of treasury stock (Note 13), shares		0
Acquisition of treasury stock (Note 13), value	(42.2)	$ 0.0	0.0	0.0	0.0	(42.2)
Dividend to common stockholders, shares		0
Dividend to common stockholders, value	28.7	$ 0.0	0.0	28.7	0.0	0.0
Net income for the period	119.3	0.0	0.0	119.3	0.0	0.0
Change in net unrealized gains on investments, net of taxes	$ (54.5)	$ 0.0	0.0	0.0	(54.5)	0.0
Stockholders' equity, shares at Dec. 31, 2021	57,690,254	57,690,254
Stockholders' equity, value at Dec. 31, 2021	$ 1,213.1	$ 0.6	$ 410.7	$ 1,338.5	$ 60.6	$ (597.3)