Summary of Significant Accounting Policies - Revenue Adjustment - Income Statement (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 240,502	$ 221,312	$ 486,527	$ 453,701
Right-to-use contract commissions, deferred, net	(262)	(112)	(286)	(196)
Operating Expenses [Abstract]
Total expenses	192,946	177,889	375,989	348,354
Consolidated net income	49,169	44,463	113,346	107,537
Net income available for Common Stockholders	$ 46,137	$ 39,498	$ 106,359	$ 96,385
Earnings per common share - Basic (usd per share)	$ 0.52	$ 0.46	$ 1.20	$ 1.12
Earnings per common share - Fully Diluted (usd per share)	$ 0.52	$ 0.45	$ 1.20	$ 1.11
Calculated under Revenue Guidance in Effect before Topic 606
Revenues:
Total revenues	$ 241,237		$ 487,996
Right-to-use contract commissions, deferred, net	(43)		164
Operating Expenses [Abstract]
Total expenses	193,165		376,439
Consolidated net income	49,685		114,365
Net income available for Common Stockholders	$ 46,629		$ 107,323
Earnings per common share - Basic (usd per share)	$ 0.53		$ 1.21
Earnings per common share - Fully Diluted (usd per share)	$ 0.53		$ 1.21
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenues:
Total revenues	$ (735)		$ (1,469)
Right-to-use contract commissions, deferred, net	219		450
Operating Expenses [Abstract]
Total expenses	(219)		(450)
Consolidated net income	(516)		(1,019)
Net income available for Common Stockholders	$ (492)		$ (964)
Earnings per common share - Basic (usd per share)	$ (0.01)		$ (0.01)
Earnings per common share - Fully Diluted (usd per share)	$ (0.01)		$ (0.01)
Right-to-use contract upfront payments, deferred, net
Revenues:
Right-to-use contract upfront payments, deferred, net	$ (2,021)	$ (1,321)	$ (3,306)	$ (2,096)
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	(1,286)		(1,837)
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	$ 735		$ 1,469