Stockholders' Equity - Narrative (Details) - USD ($)		3 Months Ended				6 Months Ended		125 Months Ended
	May 05, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized (in shares)		400,000,000		400,000,000		400,000,000	400,000,000	400,000,000	400,000,000
Common stock, par value (in dollars per share)		$ 0.01				$ 0.01		$ 0.01	$ 0.01
Preferred stock, authorized (in shares)		100,000,000				100,000,000		100,000,000
Preferred stock, par value (in dollars per share)		$ 0.01				$ 0.01		$ 0.01
Common stock aggregate offering price, remaining						$ 93,300,000
Issuance of common stock, net of issuance costs (in shares)				2,375,270			4,364,071
Shares issued, price per share (in dollars per share)		$ 52.81				$ 52.81		$ 52.81
Net proceeds of common share issuance						$ 55,492,000	$ 191,018,000
Total compensation to the applicable sales agents		$ 500,000		$ 1,600,000		$ 800,000	2,800,000
Shares repurchase program, authorized repurchase amount (in shares)		3,000,000				3,000,000		3,000,000
Shares granted to independent directors (in shares)						20,501
Grant date fair value per share of restricted stock awards (in dollars per share)						$ 58.08
Stock based compensation expense						$ 4,495,000	6,160,000
Stockholders' Equity Attributable to Parent		$ 1,589,266,000	$ 1,542,722,000	$ 1,436,178,000	$ 1,333,176,000	1,589,266,000	1,436,178,000	$ 1,589,266,000	$ 1,517,123,000	$ 1,247,797,000
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock, net of issuance costs (in shares)		630,490	562,521	2,386,470	2,184,888
Deposits to deferred compensation plan (in shares)		(3,730)	(135,494)
Stockholders' Equity Attributable to Parent		$ 684,000	$ 677,000	$ 656,000	$ 632,000	684,000	656,000	684,000	$ 673,000	$ 610,000
Common Shares Held in Deferred Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stockholders' Equity Attributable to Parent		$ 139,224,000	$ 135,494,000			$ 139,224,000		$ 139,224,000
2019 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining balance of shares available (in shares)		1,429,652				1,429,652		1,429,652
Long Term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock, net of issuance costs (in shares)						135,494
Compensation costs		$ 119,000		2,392,000		$ 725,000	3,770,000
Weighted average offering price per share (in dollars per share)						$ 54.22
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation costs		700,000		$ 400,000		$ 1,400,000	$ 900,000
Fair value of the restricted stock granted						$ 1,200,000
Vesting period for the restricted stock						3 years
Unrecognized compensation costs related to restricted stock issuances		$ 5,800,000				$ 5,800,000		$ 5,800,000
Remaining weighted average period						3 years 6 months
Restricted stock | Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted to independent directors (in shares)	11,190
Grant date fair value per share of restricted stock awards (in dollars per share)	$ 53.62
Compensation costs	$ 600,000
Restricted stock | 2019 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance as restricted stock grants under 2019 plan (in shares)		1,898,961				1,898,961		1,898,961
Minimum | Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value per share of restricted stock awards (in dollars per share)								$ 14.20
Maximum | Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value per share of restricted stock awards (in dollars per share)								58.08
$300 Million ATM Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock aggregate offering price						$ 300,000,000
Issuance of common stock, net of issuance costs (in shares)		619,300				1,046,327
Shares issued, price per share (in dollars per share)		$ 53.04		$ 45.76		$ 53.04	$ 45.76	$ 53.04
Net proceeds of common share issuance		$ 32,200,000				$ 54,700,000
$250 Million ATM Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock aggregate offering price							$ 250,000,000.0
Shares issued, price per share (in dollars per share)				$ 43.77			$ 43.77
$250 Million and $300 Million ATM Programs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net proceeds of common share issuance				$ 107,100,000			$ 188,200,000