Summary of Significant Accounting Policies - Adoption of New Accounting Standard (Details) - USD ($) $ / shares in Units, $ 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	Jan. 01, 2019	Jan. 01, 2018	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from the adoption of ASU 2014-09 (as described in Note 2)													$ (15,186)
Assets
Deferred commission expense					$ 31,443					$ 31,443
Liabilities:
Deferred revenue-upfront payment from right-to-use contracts	$ 116,363				85,596				$ 116,363	85,596	$ 81,484		108,983	$ 81,484
Equity
Distribution in excess of accumulated earnings	(211,034)				(211,980)				(211,034)	(211,980)
Revenues:
Right-to-use contract upfront payments, deferred, net										14,132
Total revenues	$ 243,451	$ 256,675	$ 240,502	$ 246,025	$ 229,986	$ 241,625	$ 221,312	$ 232,389	986,653	925,312	870,435
Expenses
Right-to-use contract commissions, deferred, net									(813)	(354)	(223)
Total expenses									765,206	718,700	685,908
Consolidated net income									226,386	210,377	187,132
Net income available for Common Stockholders									$ 212,596	$ 189,904	$ 164,037
Earnings per Common Share – Basic (usd per share)	$ 0.56	$ 0.63	$ 0.52	$ 0.68	$ 0.51	$ 0.56	$ 0.46	$ 0.66	$ 2.39	$ 2.18	$ 1.93
Earnings per Common Share – Fully Diluted (usd per share)	$ 0.56	$ 0.63	$ 0.52	$ 0.68	$ 0.51	$ 0.56	$ 0.45	$ 0.65	$ 2.38	$ 2.17	$ 1.92
Effect of Change Higher/(Lower)
Assets
Deferred commission expense													39,643
Liabilities:
Deferred revenue-upfront payment from right-to-use contracts													108,982
Equity
Distribution in excess of accumulated earnings													(227,166)
Revenues:
Total revenues									$ (2,980)
Expenses
Right-to-use contract commissions, deferred, net									868
Total expenses									(868)
Consolidated net income									(2,102)
Net income available for Common Stockholders									$ (1,989)
Earnings per Common Share – Basic (usd per share)									$ (0.02)
Earnings per Common Share – Fully Diluted (usd per share)									$ (0.02)
Adjustment due to ASU 2014-09 Adoption | Balances Without Adoption of ASU 2014-09
Assets
Deferred commission expense													8,200
Liabilities:
Deferred revenue-upfront payment from right-to-use contracts													23,386
Equity
Distribution in excess of accumulated earnings													(15,186)
Revenues:
Total revenues									$ 989,633
Expenses
Right-to-use contract commissions, deferred, net									55
Total expenses									766,074
Consolidated net income									228,488
Net income available for Common Stockholders									$ 214,585
Earnings per Common Share – Basic (usd per share)									$ 2.41
Earnings per Common Share – Fully Diluted (usd per share)									$ 2.40
Right-to-use contract upfront payments, deferred, net
Liabilities:
Deferred revenue-upfront payment from right-to-use contracts	$ 116,363				$ 85,596				$ 116,363	$ 85,596
Right-to-use contract upfront payments, deferred, net | Effect of Change Higher/(Lower)
Revenues:
Right-to-use contract upfront payments, deferred, net									2,980
Right-to-use contract upfront payments, deferred, net | Adjustment due to ASU 2014-09 Adoption | Balances Without Adoption of ASU 2014-09
Revenues:
Right-to-use contract upfront payments, deferred, net									$ (4,400)
Scenario, Forecast | Subsequent Event | Accounting Standards Update 2016-02
Expenses
Lease liability												$ 18,000
Straight-line rent liability												1,000
Operating lease asset												$ 19,000
Distributions in Excess of Accumulated Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from the adoption of ASU 2014-09 (as described in Note 2)													(15,186)
Distributions in Excess of Accumulated Earnings | Adjustment due to ASU 2014-09 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from the adoption of ASU 2014-09 (as described in Note 2)													$ 15,200