Summary of Significant Accounting Policies (Effects of New Accounting Pronouncements) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect on retained earnings										$ 2,000
Revenues	$ 621,787	$ 618,264	$ 627,533	$ 596,117	$ 646,365	$ 629,735	$ 619,130	$ 562,684	$ 2,463,701	2,457,914	$ 2,116,737
Operating expenses									2,238,229	2,233,699	1,972,917
Right-of-use asset	11,376								11,376			$ 8,700
Operating lease liabilities	$ 11,736								11,736			$ 8,700
Difference between revenue guidance in effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues									1,400	500
Operating expenses									1,000	700
Other revenues
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues									64,439	59,662	$ 64,683
Other revenues | Difference between revenue guidance in effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues									$ (14,100)	$ (14,300)