Incentive Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation [Abstract]
Number of common shares available for grant under the incentive plan (in shares)	2,881,839
Share-based compensation	$ 24,345	$ 16,643	$ 13,345
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost	2,300
Tax benefits (expense) associated with stock-based compensation	$ 15,400	$ 7,700	5,300
Stock option activity [Roll Forward]
Outstanding, beginning balance (in shares)	16,000
Granted (in shares)	0
Exercised (in shares)	0
Canceled (in shares)	0
Outstanding, ending balance (in shares)	16,000	16,000
Exercisable, ending balance (in shares)	16,000
Weighted average exercise price [Abstract]
Outstanding, beginning balance (in dollars per share)	$ 15.30
Granted (in dollars per share)	0.00
Exercised (in dollars per share)	0.00
Canceled (in dollars per share)	0.00
Outstanding, ending balance (in dollars per share)	15.30	$ 15.30
Exercisable, ending balance (in dollars per share)	$ 15.30
Weighted average remaining contractual life, stock option awards outstanding at end of year (in years)	3 years 6 months
Weighted average remaining contractual life, stock option awards exercisable at end of year (in years)	3 years 6 months
Aggregate intrinsic value of options outstanding, ending balance	$ 657
Aggregate intrinsic value of options exercisable ending balance	$ 657
Intrinsic value of options exercised during the year		$ 800	$ 300
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards vesting period	3 years
Employee Stock Purchase Plan (ESPP) [Abstract]
ESPP Stock Issued During Period (in shares)	38,000	34,000	48,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of shares vested during the year	$ 46,000	$ 16,200	$ 18,600
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 12,100
Unrecognized compensation expense, period for recognition (in months)	20 months
Equity Instruments Other than Options, Nonvested, Number of Shares[Roll Forward]
Balance at 12/31/2016 (in shares)	1,006,000
Granted (in shares)	323,000
Vested (in shares)	(952,000)
Canceled (in shares)	(34,000)
Balance at 12/31/2017 (in shares)	343,000	1,006,000
Weighted Average Grant Date Fair Value, Equity Instruments Other than Options [Abstract]
Balance at 12/31/2016 (in dollars per share)	$ 23.09
Granted (in dollars per share)	42.15	$ 26.33	$ 25.77
Vested (in dollars per share)	24.87
Canceled (in dollars per share)	31.24
Balance at 12/31/2017 (in dollars per share)	$ 35.29	23.09
Restricted Stock [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards vesting period	3 years
Restricted Stock [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards vesting period	5 years
Long-Term Incentive Program Awards [Member]
Weighted Average Grant Date Fair Value, Equity Instruments Other than Options [Abstract]
Balance at 12/31/2016 (in dollars per share)	$ 28.02
Granted (in dollars per share)	41.55
Vested (in dollars per share)	0.00
Canceled (in dollars per share)	0.00
Balance at 12/31/2017 (in dollars per share)	$ 32.03	$ 28.02
Long-Term Incentive Program Awards [Member] | Minimum [Member]
Equity Instruments Other than Options, Nonvested, Number of Shares[Roll Forward]
Balance at 12/31/2016 (in shares)	463,000
Granted (in shares)	195,000
Vested (in shares)	0
Canceled (in shares)	0
Balance at 12/31/2017 (in shares)	658,000	463,000
NumberofShareExpectedtoVestBasedOnCurrentEstimatesofPerformanceAchievment	926,000
Long-Term Incentive Program Awards [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards vesting period	3 years
Equity Instruments Other than Options, Nonvested, Number of Shares[Roll Forward]
Balance at 12/31/2016 (in shares)	844,000
Granted (in shares)	332,000
Vested (in shares)	0
Canceled (in shares)	0
Balance at 12/31/2017 (in shares)	1,176,000	844,000
2015LTIPAward [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 500
Equity Instruments Other than Options, Nonvested, Number of Shares[Roll Forward]
NumberofShareExpectedtoVestBasedOnCurrentEstimatesofPerformanceAchievment	358,000
2016LTIPAwards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 3,300
Equity Instruments Other than Options, Nonvested, Number of Shares[Roll Forward]
NumberofShareExpectedtoVestBasedOnCurrentEstimatesofPerformanceAchievment	317,000
2017LTIPAwards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 7,600
Equity Instruments Other than Options, Nonvested, Number of Shares[Roll Forward]
NumberofShareExpectedtoVestBasedOnCurrentEstimatesofPerformanceAchievment	251,000