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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Aggregate dividends paid during the period for each share of common stock outstanding.

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Aggregate dividends declared during the period for each share of common stock outstanding.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Amount of paid and unpaid preferred stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

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Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity.

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The cash outflow for cost incurred directly with the issuance of an equity security.

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The percentage rate used to calculate dividend payments on preferred stock.

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Aggregate dividends paid during the period for each share of preferred stock outstanding.

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The cash inflow from the additional capital contribution to the entity.

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Number of shares issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the shareholders to reinvest dividends paid to them by the entity on new issues of stock by the entity.

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Number of new stock issued during the period.

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Equity impact of the value of new stock issued during the period. Includes shares issued in an initial public offering or a secondary public offering.

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Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Feb. 28, 2018
Class of Stock [Line Items]
Total allowable shares (in shares)		375,000,000		375,000,000		375,000,000
Net proceeds				$ 132,191	$ 74,503
Total shares issued (in shares)		156,990,141		156,990,141		153,577,028
Stock issuance costs				$ 1,740	1,238
Common stock Dividends		$ 72,850	$ 67,660	$ 145,583	$ 134,440
Common stock Per share (in dollars per share)		$ 0.475	$ 0.455	$ 0.950	$ 0.910
Subsequent Event
Class of Stock [Line Items]
Common stock, dividends, per share, declared (in dollars per share)	$ 0.500
Series D Preferred Stock
Class of Stock [Line Items]
Dividends		$ 0	$ 0	$ 0	$ 3,598
Per depository share (in dollars per share)		$ 0	$ 0	$ 0	$ 0.312847
Dividend rate				6.625%
Series E Preferred Stock
Class of Stock [Line Items]
Dividends		$ 4,096	$ 4,096	$ 8,194	$ 8,194
Per depository share (in dollars per share)		$ 0.356250	$ 0.356250	$ 0.712500	$ 0.712500
Dividend rate				5.70%		5.70%
Series F Preferred Stock
Class of Stock [Line Items]
Dividends		$ 4,485	$ 4,485	$ 8,970	$ 8,970
Per depository share (in dollars per share)		$ 0.325000	$ 0.325000	$ 0.650000	$ 0.650000
Dividend rate				5.20%		5.20%
DRIP
Class of Stock [Line Items]
Total allowable shares (in shares)							10,000,000
Shares of common stock (in shares)				143,997	34,466
Net proceeds				$ 5,471	$ 1,413
2018 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)		12,000,000		12,000,000
Total shares issued (in shares)		3,037,464		3,037,464
2016 ATM
Class of Stock [Line Items]
Total allowable shares (in shares)		12,000,000		12,000,000
Total shares issued (in shares)		10,044,656		10,044,656
ATM Equity Programs
Class of Stock [Line Items]
Shares of common stock (in shares)				3,037,464	1,653,155
Average price per share (net) (in dollars per share)				$ 41.17	$ 43.64
Net proceeds				$ 125,052	$ 72,139
Stock issuance costs				$ 1,633	$ 1,183