Stock-Based Compensation (Nonvested Performance-based Award Activity) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Incentive Stock Awards [Member]
Non-vested PVS Awards [Roll Forward]
Granted at target level	1,500	1,800	2,400
Forfeited	(200)	(800)	(800)
Non-vested PVS Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Granted at target level	$ 93.00	$ 86.93	$ 59.64
Performance-Vesting Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares to be issued upon conversion	108,626
Average remaining term of shares issued at conversion	1 year
Non-vested PVS Awards [Roll Forward]
Outstanding, Beginning	341,944	378,062	422,726
Granted at target level	102,307	92,045	115,035
Adjustments above/(below) target	2,284	11,369	(19,339)
Vested and converted	(121,984)	(116,684)	(173,364)
Forfeited	(23,946)	(110)	(5,674)
Outstanding, Ending	296,037	341,944	378,062
Non-vested PVS Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, Beginning (in usd per share)	$ 64.38	$ 54.47	$ 45.60
Granted at target level	90.45	84.01	60.47
Adjustments above/(below) target	33.86	42.85	38.71
Vested and converted	93.00	50.06	59.64
Forfeited	68.65	73.64	49.86
Outstanding, Ending (in usd per share)	$ 76.84	$ 64.38	$ 54.47
Performance-Vesting Units [Member]
Non-vested PVS Awards [Roll Forward]
Outstanding, Beginning	1,972	2,451	29,196
Granted at target level	560	598	419
Adjustments above/(below) target	30	107	(2,858)
Vested and converted	(910)	(970)	(29,032)
Forfeited	0	0	(990)
Outstanding, Ending	1,592	1,972	2,451
Non-vested PVS Awards, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, Beginning (in usd per share)	$ 92.25	$ 25.28	$ 32.07
Granted at target level	89.64	83.47	59.64
Adjustments above/(below) target	41.53	66.61	30.80
Vested and converted	93.00	86.93	59.64
Forfeited	0.00	0.00	50.55
Outstanding, Ending (in usd per share)	$ 79.48	$ 92.25	$ 25.28
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%