Share-Based Compensation - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) plan shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock incentive plans | plan	2
Shares available for issuance (in shares) | shares	2,761,369
Stock option granted (in shares) | shares	0	0	0
Aggregate intrinsic value of stock options exercised	$ 11.2	$ 6.4	$ 4.6
Tax benefit from stock option exercised	4.4	2.5	1.8
Fair value of options vested during period	0.6	1.8	1.8
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost charged against income	0.4	1.3	1.6
Income tax benefit recognized during the period	0.2	0.5	0.6
Nonvested Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost charged against income	7.0	6.0	7.5
Income tax benefit recognized during the period	$ 2.8	$ 2.3	2.9
Shares of non-vested common stock granted (in shares) | shares	135,619	131,000
Total unrecognized compensation cost related to stock options	$ 17.4
Weighted-average period of recognition of unrecognized compensation cost	2 years 11 months 5 days
Nonvested Stock Awards | Board Of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum vesting period of outstanding options and non vested shares (years)	3 years
Shares of non-vested common stock granted (in shares) | shares	4,992
Nonvested Stock Awards | Employees - Non Vested Shares One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of non-vested common stock granted (in shares) | shares	130,627
Nonvested Stock Awards | Employees - Non Vested Shares Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum vesting period of outstanding options and non vested shares (years)	4 years
Nonvested Stock Awards | Anniversary One | Board Of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	33.33%
Nonvested Stock Awards | Anniversary One | Employees - Non Vested Shares Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	25.00%
Nonvested Stock Awards | Anniversary Two | Board Of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	33.33%
Nonvested Stock Awards | Anniversary Two | Employees - Non Vested Shares Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	25.00%
Nonvested Stock Awards | Anniversary Three | Board Of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	33.33%
Nonvested Stock Awards | Anniversary Three | Employees - Non Vested Shares Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	25.00%
Nonvested Stock Awards | Anniversary Four | Employees - Non Vested Shares Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	25.00%
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost charged against income	$ 11.3	$ 9.0	8.1
Income tax benefit recognized during the period	4.4	$ 3.5	$ 3.2
Unrecognized compensation cost	$ 17.6
Weighted-average period of recognition of unrecognized compensation cost	3 years 3 months 18 days
Restricted stock units granted (in shares) | shares	229,590	250,000
Restricted Stock Units (RSUs) | Executive and Non-Executive
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum vesting period of outstanding options and non vested shares (years)	4 years
Shares authorized by right (in shares) | shares	1
Award performance period	3 years
Restricted Stock Units (RSUs) | Anniversary One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	25.00%
Restricted Stock Units (RSUs) | Anniversary Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	25.00%
Restricted Stock Units (RSUs) | Anniversary Three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	25.00%
Restricted Stock Units (RSUs) | Anniversary Four
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights, percentage	25.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum vesting period of outstanding options and non vested shares (years)	3 years
Expiration period of outstanding options (years)	5 years
Minimum | Nonvested Stock Awards | Board Of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum vesting period of outstanding options and non vested shares (years)	3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum vesting period of outstanding options and non vested shares (years)	4 years
Expiration period of outstanding options (years)	10 years
Maximum | Nonvested Stock Awards | Board Of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum vesting period of outstanding options and non vested shares (years)	4 years