Summary of Significant Accounting Policies (Revenue Adjustment - Income Statement) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues:
Right-to-use contract upfront payments, deferred, net	$ (1,285)	$ (775)
Total revenues	246,025	232,389
Operating Expenses [Abstract]
Right-to-use contract commissions, deferred, net	(24)	(84)
Total expenses	183,043	170,464
Consolidated net income	64,177	63,075
Net income available for Common Stockholders	$ 60,222	$ 56,888
Earnings per common share - Basic (usd per share)	$ 0.68	$ 0.66
Earnings per common share - Fully Diluted (usd per share)	$ 0.68	$ 0.65
Calculated under Revenue Guidance in Effect before Topic 606
Revenues:
Right-to-use contract upfront payments, deferred, net	$ (551)
Total revenues	246,759
Operating Expenses [Abstract]
Right-to-use contract commissions, deferred, net	207
Total expenses	183,274
Consolidated net income	64,680
Net income available for Common Stockholders	$ 60,694
Earnings per common share - Basic (usd per share)	$ 0.69
Earnings per common share - Fully Diluted (usd per share)	$ 0.68
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenues:
Right-to-use contract upfront payments, deferred, net	$ 734
Total revenues	734
Operating Expenses [Abstract]
Right-to-use contract commissions, deferred, net	231
Total expenses	231
Consolidated net income	503
Net income available for Common Stockholders	$ 472
Earnings per common share - Basic (usd per share)	$ 0.01
Earnings per common share - Fully Diluted (usd per share)	$ 0.00