Summary of Significant Accounting Policies - Revenue Adjustment - Income Statement (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Total revenues	$ 256,675	$ 241,625	$ 743,202	$ 695,326
Expenses
Right-to-use contract commissions, deferred, net	(458)	(176)	(744)	(372)
Total expenses	197,803	187,446	573,792	535,799
Consolidated net income	59,660	54,865	173,006	162,403
Net income available for Common Stockholders	$ 56,070	$ 48,525	$ 162,429	$ 144,911
Earnings per common share - Basic (usd per share)	$ 0.63	$ 0.56	$ 1.83	$ 1.67
Earnings per common share - Fully Diluted (usd per share)	$ 0.63	$ 0.56	$ 1.82	$ 1.66
Calculated under Revenue Guidance in Effect before Topic 606
Revenues:
Total revenues	$ 257,427		$ 745,423
Expenses
Right-to-use contract commissions, deferred, net	(245)		(81)
Total expenses	198,016		574,455
Consolidated net income	60,189		174,554
Net income available for Common Stockholders	$ 56,567		$ 163,890
Earnings per common share - Basic (usd per share)	$ 0.63		$ 1.85
Earnings per common share - Fully Diluted (usd per share)	$ 0.63		$ 1.84
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenues:
Total revenues	$ (752)		$ (2,221)
Expenses
Right-to-use contract commissions, deferred, net	213		663
Total expenses	(213)		(663)
Consolidated net income	(529)		(1,548)
Net income available for Common Stockholders	$ (497)		$ (1,461)
Earnings per common share - Basic (usd per share)	$ 0.00		$ (0.02)
Earnings per common share - Fully Diluted (usd per share)	$ 0.00		$ (0.02)
Right-to-use contract upfront payments, deferred, net
Revenues:
Right-to-use contract upfront payments, deferred, net	$ (2,883)	$ (1,670)	$ (6,189)	$ (3,766)
Right-to-use contract upfront payments, deferred, net | Calculated under Revenue Guidance in Effect before Topic 606
Revenues:
Right-to-use contract upfront payments, deferred, net	(2,131)		(3,968)
Right-to-use contract upfront payments, deferred, net | 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	$ 752		$ 2,221