Stock-Based Incentive Plans (Details) $ / shares in Units, $ in Thousands				3 Months Ended	9 Months Ended		12 Months Ended
	Apr. 21, 2015 shares	Apr. 20, 2015 $ / shares	Feb. 02, 2015 $ / shares shares	Mar. 31, 2015 $ / shares shares	Sep. 30, 2015 USD ($) plan $ / shares shares	Sep. 30, 2014 USD ($)	Dec. 31, 2014 $ / shares shares
Stock-Based Compensation
Number of stock-based incentive plans | plan					1
Number of stock option plans superseded | plan					2
Aggregate Intrinsic Value
Granted (in shares)					0		0
Stock options outstanding and expected to vest					0
Total intrinsic value of options exercised (in dollars) | $					$ 1,600	$ 700
Unrecognized compensation cost and vesting period
Unrecognized compensation costs related to unvested share-based compensation arrangements | $					$ 17,400
Weighted-average period for recognition of unrecognized stock-based compensation expense					1 year 11 months 23 days
Restricted Stock Units
Stock-Based Compensation
Weighted average granted date fair value (in dollars per share) | $ / shares		$ 36.33	$ 32.64
Restricted stock unit activity
Outstanding at the beginning of the period (in shares)				504,000	504,000
Awarded (in shares)			330,497		351,000
Vested (in shares)					(317,000)
Forfeited (in shares)					(2,000)
Outstanding at the end of the period (in shares)					536,000		504,000
Outstanding and expected to vest at end of the period (in shares)					524,000
Weighted-Average Exercise Price
Weighted-average exercise price at beginning of the period (in dollars per share) | $ / shares				$ 31.67	$ 31.67
Weighted-average exercise price at end of the period (in dollars per share) | $ / shares					31.56		$ 31.67
Outstanding and expected to vest at the end of the period (in dollars per share) | $ / shares					$ 31.56
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars) | $					$ 17,950
Outstanding and expected to vest at end of the period (in dollars) | $					$ 17,541
Aggregate Intrinsic Value
Closing price per share (in dollars per share) | $ / shares					$ 33.49
Total intrinsic value of awards vested (in dollars) | $					$ 10,100	$ 9,000
Non-Qualified Stock Options
Non-Qualified Stock Options activity
Exercisable at the start of the period (in shares)				855,000	855,000
Exercised (in shares)					(238,000)
Forfeited (in shares)					(2,000)
Exercisable at the end of the period (in shares)					615,000		855,000
Weighted-Average Exercise Price
Weighted-average exercise price at beginning of the period (in dollars per share) | $ / shares				$ 29.48	$ 29.48
Weighted-average exercise price at end of the period (in dollars per share) | $ / shares					$ 29.51		$ 29.48
Weighted-average remaining contractual life					2 years 3 months 18 days
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars) | $					$ 2,447
Unvested Stock Options
Number of Shares, Unvested Stock Options
Vested (in shares)				(99,000)
Unvested at the end of the period (in shares)					0
Weighted-Average Grant-Date Fair Value, Unvested Stock Options
Vested (in dollars per share) | $ / shares				$ 10.33
Non Employee Directors | Restricted Stock Units
Restricted stock unit activity
Awarded (in shares)	1,950		8,550