Share-Based Compensation - Non-employee Director Option Plan (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement By Share-based Payment Award, Fair Value Assumptions, Percent Expected To Vest		75.10%	74.80%
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		190,020	233,400	219,400
Weighted Average Grant Date Fair Value	$ 87.12	$ 117.47		$ 79.38
Director Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares Authorized		375,000
Number of Shares Available for Grant		191,774
Two Thousand Fifteen Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares Authorized	1,750,000
Number of Shares Available for Grant		974,864
Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares			16,500
Weighted Average Grant Date Fair Value			$ 88.30
Executive Officer [Member] | Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award Vesting Rights, Percentage		100.00%	100.00%
Director [Member] | 2004 Non-employee Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		18,000	16,800
Weighted Average Grant Date Fair Value		$ 113.68	$ 85.28
Award Vesting Rights, Percentage		100.00%	100.00%
Share-based Payment Arrangement, Tranche One [Member] | Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award Vesting Rights, Percentage			35.00%
Share-based Payment Arrangement, Tranche Two [Member] | Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares			50,100
Weighted Average Grant Date Fair Value			$ 86.97
Award Vesting Rights, Percentage			35.00%
Share-based Payment Arrangement, Tranche Three [Member] | Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award Vesting Rights, Percentage			20.00%
Share-based Compensation Award, Tranche Four [Member] | Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award Vesting Rights, Percentage			5.00%
Share-based Compensation Award, Tranche Five [Member] | Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award Vesting Rights, Percentage			5.00%
Time Based [Member] | Executive Officer [Member] | Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		44,000	60,000
Weighted Average Grant Date Fair Value		$ 115.39	$ 87.24
Market Condition [Member] | Executive Officer [Member] | Two Thousand Fifteen Equity Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		66,000	90,000
Weighted Average Grant Date Fair Value		$ 115.39	$ 65.24
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Price		$ 115.39	$ 87.24
Award Date One [Member] | Key Employees [Member] | 2004 Non-employee Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		55,770
Weighted Average Grant Date Fair Value		$ 120.01
Award Date Two [Member] | Key Employees [Member] | 2004 Non-employee Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		6,250
Weighted Average Grant Date Fair Value		$ 142.48