Summary of Significant Accounting Policies (Effects of New Accounting Pronouncements) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect on retained earnings									$ 2,000
Revenues	$ 646,365	$ 629,735	$ 619,130	$ 562,684	$ 567,365	$ 528,643	$ 519,508	$ 501,221	2,457,914	$ 2,116,737	$ 2,008,991
Operating expenses									2,233,699	1,972,917	1,882,921
Prior period reclassification adjustment										1,000	2,700
Difference between revenue guidance in effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues									500
Operating expenses									700
Other revenues
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues									59,662	$ 64,683	$ 58,811
Other revenues | Difference between revenue guidance in effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues									$ (14,300)