Capital Stock - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 28, 2017	Oct. 31, 2015	Jun. 30, 2014
Equity Class Of Treasury Stock [Line Items]
Authorized amount to repurchase outstanding common stock				$ 300,000,000	$ 250,000,000
Remaining authorized amount to repurchase outstanding common stock	$ 242,500,000
Total cost of shares repurchased	150,300,000	$ 111,100,000	$ 146,100,000
Dividends paid on common stock	$ 42,600,000	$ 39,800,000	$ 38,300,000
Dividends per share of common stock	$ 0.47	$ 0.44	$ 0.40
2014 Repurchase Plan
Equity Class Of Treasury Stock [Line Items]
Authorized amount to repurchase outstanding common stock						$ 150,000,000