Debt - Components of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Long-Term Debt
Long-term debt principal	$ 4,309.3	$ 4,311.1
Net unamortized discount, premium and debt issuance costs	(29.1)	(31.9)
Long-term debt carrying amount	$ 4,280.2	$ 4,279.2
3.3% notes payable, due 2022
Long-Term Debt
Interest rate (as a percent)	3.30%	3.30%
Long-term debt principal	$ 300.0	$ 300.0
Net unamortized discount, premium and debt issuance costs	(0.3)	(0.7)
Long-term debt carrying amount	$ 299.7	$ 299.3
3.125% notes payable, due 2023
Long-Term Debt
Interest rate (as a percent)	3.125%	3.125%
Long-term debt principal	$ 300.0	$ 300.0
Net unamortized discount, premium and debt issuance costs	(0.4)	(0.7)
Long-term debt carrying amount	$ 299.6	$ 299.3
3.4% notes payable, due 2025
Long-Term Debt
Interest rate (as a percent)	3.40%	3.40%
Long-term debt principal	$ 400.0	$ 400.0
Net unamortized discount, premium and debt issuance costs	(1.7)	(2.2)
Long-term debt carrying amount	$ 398.3	$ 397.8
3.1% notes payable, due 2026
Long-Term Debt
Interest rate (as a percent)	3.10%	3.10%
Long-term debt principal	$ 350.0	$ 350.0
Net unamortized discount, premium and debt issuance costs	(1.9)	(2.1)
Long-term debt carrying amount	$ 348.1	$ 347.9
3.7% notes payable, due 2029
Long-Term Debt
Interest rate (as a percent)	3.70%	3.70%
Long-term debt principal	$ 500.0	$ 500.0
Net unamortized discount, premium and debt issuance costs	(5.0)	(5.5)
Long-term debt carrying amount	$ 495.0	$ 494.5
2.125% notes payable, due 2030
Long-Term Debt
Interest rate (as a percent)	2.125%	2.125%
Long-term debt principal	$ 600.0	$ 600.0
Net unamortized discount, premium and debt issuance costs	(4.1)	(4.6)
Long-term debt carrying amount	$ 595.9	$ 595.4
6.05% notes payable, due 2036
Long-Term Debt
Interest rate (as a percent)	6.05%	6.05%
Long-term debt principal	$ 505.6	$ 505.6
Net unamortized discount, premium and debt issuance costs	(2.3)	(2.5)
Long-term debt carrying amount	$ 503.3	$ 503.1
4.625% notes payable, due 2042
Long-Term Debt
Interest rate (as a percent)	4.625%	4.625%
Long-term debt principal	$ 300.0	$ 300.0
Net unamortized discount, premium and debt issuance costs	(3.0)	(3.1)
Long-term debt carrying amount	$ 297.0	$ 296.9
4.35% notes payable, due 2043
Long-Term Debt
Interest rate (as a percent)	4.35%	4.35%
Long-term debt principal	$ 300.0	$ 300.0
Net unamortized discount, premium and debt issuance costs	(3.0)	(3.1)
Long-term debt carrying amount	$ 297.0	$ 296.9
4.3% notes payable, due 2046
Long-Term Debt
Interest rate (as a percent)	4.30%	4.30%
Long-term debt principal	$ 300.0	$ 300.0
Net unamortized discount, premium and debt issuance costs	(3.1)	(3.2)
Long-term debt carrying amount	296.9	296.8
Floating rate notes payable, due 2055
Long-Term Debt
Long-term debt principal	400.0	400.0
Net unamortized discount, premium and debt issuance costs	(4.6)	(4.7)
Long-term debt carrying amount	395.4	395.3
Non-recourse mortgages and notes payable
Long-Term Debt
Long-term debt principal	53.7	55.5
Net unamortized discount, premium and debt issuance costs	0.3	0.5
Long-term debt carrying amount	$ 54.0	$ 56.0

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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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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