Income Taxes - Effective Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Statutory U.S. federal income tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	2.90%	1.80%	1.50%
Benefits and taxes related to foreign operations	(0.50%)	(3.90%)	(6.90%)
Excess tax benefit on share-based compensation	(1.80%)	(1.50%)	(2.30%)
Unrecognized tax positions	(0.40%)	(0.20%)	0.30%
Transition Tax imposed on unrepatriated foreign earnings	(0.10%)	14.10%	0.00%
Write-down of net deferred tax assets due to U.S. rate change	0.40%	5.30%	0.00%
Other	(0.70%)	0.30%	0.40%
Effective income tax rates	20.80%	50.90%	28.00%