Business Segments (Revenue by Geographic Area) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 255,965	$ 270,615	$ 258,026	$ 256,573	$ 240,670	$ 216,763	$ 215,052	$ 209,575	$ 1,041,179	$ 882,060	$ 806,247
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									903,511	769,732	704,360
Rest of world
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues									$ 137,668	$ 112,328	$ 101,887