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Maximum Times Series B Receives No Dividends in Order for Voting Rights

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The number of additional directors that can be elected by the preferred stock holders on getting the voting rights.

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The minimum percentage of preferred stock holders' affirmative vote necessary to make certain material and adverse changes to the terms of the preferred stock.

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Shares Outstanding for Future Offerings

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Aggregate number of common shares reserved for future issuance.

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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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Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding.

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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 declared during the period for each share of preferred stock outstanding.

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The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share.

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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Cash received on stock transaction after deduction of issuance costs.

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The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

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

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The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

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The remaining number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

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Stockholders' Equity (Narrative) (Details)								3 Months Ended				9 Months Ended		12 Months Ended	187 Months Ended
Stockholders' Equity (Narrative) (Details)	Sep. 12, 2019 $ / shares	Aug. 09, 2019 $ / shares	Jun. 12, 2019 $ / shares	May 20, 2019 $ / shares	Mar. 06, 2019 $ / shares	Feb. 15, 2019 $ / shares	Apr. 15, 2013 $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 $ / shares	Sep. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 $ / shares	Sep. 30, 2019 USD ($) director quarter $ / shares shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 $ / shares shares	Mar. 31, 2019 USD ($) shares	Aug. 16, 2019 USD ($)	Sep. 16, 2016 shares	Dec. 31, 2013 shares	Aug. 31, 2005 shares
Stockholders' Equity
Preferred stock, shares issued								8,000,000				8,000,000		8,000,000
Preferred Stock, dividend rate												7.50%		7.50%
Preferred stock, par value (in dollars per share) \| $ / shares								$ 0.01				$ 0.01		$ 0.01
Maximum quarters without dividends to get voting rights, in quarters \| quarter												6
Common stock, cash dividends declared (in dollars per share) \| $ / shares	$ 0.20		$ 0.20		$ 0.20			$ 0.20	$ 0.20	$ 0.20	$ 0.20
Dividends and dividend equivalents declared and unpaid \| $								$ 90,614,000		$ 90,136,000		$ 90,614,000	$ 90,136,000
Aggregate number of shares of common stock authorized (in shares)								886,950,000				886,950,000		886,950,000
Issuance costs \| $												$ 0	$ 350,000
Number of shares authorized to be repurchased under the Repurchase Program (in shares)																		10,000,000.0	4,000,000.0
Number of remaining shares authorized to be repurchased under the Repurchase Program (in shares)								6,616,355				6,616,355
DRSPP
Stockholders' Equity
Aggregate number of shares of common stock authorized (in shares)																	15,000,000.0
Shares of common stock authorized and available for issuance (in shares)								11,600,000				11,600,000
Common shares issued through DRSPP (in shares)								87,886				240,741			34,296,621
Net proceeds from shares issued through DRSPP \| $								$ 634,000				$ 1,800,000			$ 286,000,000.0
Series B
Stockholders' Equity
Dividend declared per share, preferred stock (in dollars per share) \| $ / shares		$ 0.46875		$ 0.46875		$ 0.46875		$ 0.46875	$ 0.46875	$ 0.46875	$ 0.46875
Preferred Stock \| Series B
Stockholders' Equity
Preferred stock, shares issued							8,000,000.0
Preferred Stock, dividend rate							7.50%					7.50%	7.50%
Preferred stock, par value (in dollars per share) \| $ / shares							$ 0.01
Preferred stock, liquidation preference (in dollars per share) \| $ / shares							25.00	$ 25.00				$ 25.00		$ 25
Preferred stock, redemption price (in dollars per share) \| $ / shares							$ 25.00
Number of additional directors that can be elected by preferred stock holders \| director												2
Minimum percentage of preferred shareholders required for approval (percent)							66.67%
At The Market
Stockholders' Equity
At-the-market, maximum potential proceeds \| $																$ 400,000,000.0
Number of shares issued (in shares)								982,350
Proceeds from sale of stock \| $								$ 7,100,000
Issuance costs \| $								72,000
Value remaining outstanding for future offerings \| $								$ 392,800,000				$ 392,800,000
Weighted Average \| At The Market
Stockholders' Equity
Gross Proceeds Per Share (USD per share) \| $ / shares								$ 7.29				$ 7.29