Debt Obligations, Debt Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Scheduled Maturities of Debt [Abstract]
2018	$ 2,855.7
2019	1,500.0
2020	1,500.0
2021	575.0
2022	650.0
Thereafter	17,699.4
Total	24,780.1	$ 23,901.6
Commercial Paper Notes [Member]
Scheduled Maturities of Debt [Abstract]
2018	1,755.7
2019	0.0
2020	0.0
2021	0.0
2022	0.0
Thereafter	0.0
Total	1,755.7
Senior Notes [Member]
Scheduled Maturities of Debt [Abstract]
2018	1,100.0
2019	1,500.0
2020	1,500.0
2021	575.0
2022	650.0
Thereafter	14,525.0
Total	19,850.0
Junior Subordinated Notes [Member]
Scheduled Maturities of Debt [Abstract]
2018	0.0
2019	0.0
2020	0.0
2021	0.0
2022	0.0
Thereafter	3,174.4
Total	$ 3,174.4

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
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-Section S99
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

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