Debt Obligations - Consolidated Debt Obligations Outstanding (Details) - USD ($)	9 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Consolidated debt obligations
Principal balance outstanding	$ 1,828,610,000
Plus: original issue premium (discount), net	(8,675,000)
Less: deferred financing costs	(6,696,000)
Debt, net	1,830,589,000
Unsecured Debt
Consolidated debt obligations
Principal balance outstanding	1,828,610,000	$ 1,828,610,000
Plus: original issue premium (discount), net	8,675,000	10,137,000
Less: deferred financing costs	(6,696,000)	(8,152,000)
Debt, net	1,830,589,000	1,830,595,000
Unsecured Debt \| $600 million Unsecured Credit Facility
Consolidated debt obligations
Debt instrument, face amount	600,000,000
Principal balance outstanding	$ 0	0
Unsecured Debt \| $600 million Unsecured Credit Facility \| LIBOR
Consolidated debt obligations
Spread on variable rate	1.10%
Unsecured Debt \| Seven-Year Term Loan - Swapped to fixed
Consolidated debt obligations
Debt instrument, term	7 years
Principal balance outstanding	$ 250,000,000	250,000,000
Effective interest rate	2.87%
Unsecured Debt \| $350.0M 3.95% Guaranteed Notes due 2023
Consolidated debt obligations
Debt instrument, face amount	$ 350,000,000.0
Debt instrument, interest rate, stated percentage	3.95%
Principal balance outstanding	$ 350,000,000	350,000,000
Effective interest rate	3.87%
Unsecured Debt \| $350.0M 4.10% Guaranteed Notes due 2024
Consolidated debt obligations
Debt instrument, face amount	$ 350,000,000.0
Debt instrument, interest rate, stated percentage	4.10%
Principal balance outstanding	$ 350,000,000	350,000,000
Effective interest rate	3.78%
Unsecured Debt \| $450.0M 3.95% Guaranteed Notes due 2027
Consolidated debt obligations
Debt instrument, face amount	$ 450,000,000.0
Debt instrument, interest rate, stated percentage	3.95%
Principal balance outstanding	$ 450,000,000	450,000,000
Effective interest rate	4.03%
Unsecured Debt \| $350.0M 4.55% Guaranteed Notes due 2029
Consolidated debt obligations
Debt instrument, face amount	$ 350,000,000.0
Debt instrument, interest rate, stated percentage	4.55%
Principal balance outstanding	$ 350,000,000	350,000,000
Effective interest rate	4.30%
Unsecured Debt \| Indenture IA (Preferred Trust I)
Consolidated debt obligations
Principal balance outstanding	$ 27,062,000	27,062,000
Unsecured Debt \| Indenture IA (Preferred Trust I) \| LIBOR
Consolidated debt obligations
Spread on variable rate	1.25%
Unsecured Debt \| Indenture IB (Preferred Trust I)
Consolidated debt obligations
Principal balance outstanding	$ 25,774,000	25,774,000
Unsecured Debt \| Indenture IB (Preferred Trust I) \| LIBOR
Consolidated debt obligations
Spread on variable rate	1.25%
Unsecured Debt \| Indenture II (Preferred Trust II)
Consolidated debt obligations
Principal balance outstanding	$ 25,774,000	$ 25,774,000
Unsecured Debt \| Indenture II (Preferred Trust II) \| LIBOR
Consolidated debt obligations
Spread on variable rate	1.25%

Consolidated Debt Obligations Abstract.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Face (par) amount of debt instrument at time of issuance.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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