Stock-Based Compensation (Nonvested Performance-based Award Activity) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Incentive Stock Awards [Member]
Non-vested PVS Awards [Roll Forward]
Granted at target level	1,800	2,400	1,500
Forfeited	(800)	(800)	(200)
Non-vested PVS Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Granted at target level	$ 86.93	$ 59.64	$ 51.53
Performance-Vesting Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares to be issued upon conversion	166,439
Average remaining term of shares issued at conversion	1 year
Non-vested PVS Awards [Roll Forward]
Outstanding, Beginning	378,062	422,726	470,719
Granted at target level	92,045	115,035	147,908
Adjustments above/(below) target	11,369	(19,339)	(132,444)
Vested and converted	(116,684)	(173,364)	(318,337)
Forfeited	(110)	(5,674)	(10,008)
Outstanding, Ending	341,944	378,062	422,726
Non-vested PVS Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, Beginning (in usd per share)	$ 54.47	$ 45.60	$ 30.93
Granted at target level	84.01	60.47	55.49
Adjustments above/(below) target	42.85	38.71	22.97
Vested and converted	50.06	59.64	51.53
Forfeited	73.64	49.86	41.84
Outstanding, Ending (in usd per share)	$ 64.38	$ 54.47	$ 45.60
Performance-Vesting Units [Member]
Non-vested PVS Awards [Roll Forward]
Outstanding, Beginning	2,451	29,196	55,509
Granted at target level	598	419	1,386
Adjustments above/(below) target	107	(2,858)	(19,315)
Vested and converted	(970)	(29,032)	(47,014)
Forfeited	0	(990)	0
Outstanding, Ending	1,972	2,451	29,196
Non-vested PVS Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, Beginning (in usd per share)	$ 25.28	$ 32.07	$ 26.15
Granted at target level	83.47	59.64	54.14
Adjustments above/(below) target	66.61	30.80	22.07
Vested and converted	86.93	59.64	51.53
Forfeited	0.00	50.55	0.00
Outstanding, Ending (in usd per share)	$ 92.25	$ 25.28	$ 32.07
Minimum [Member] | Performance-Vesting Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Actual payout range	0.00%
Minimum [Member] | Performance-Vesting Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Actual payout range	0.00%
Maximum [Member] | Performance-Vesting Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Actual payout range	200.00%
Maximum [Member] | Performance-Vesting Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Actual payout range	200.00%