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AFS, Debt Maturities (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Mar. 31, 2017
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Schedule of available-for-sale securities, debt maturities [Line Items] |  |
Available-for-sale, adjusted cost | $ 1,293.2 | $ 501.0
Available-for-sale, gross unrealized gains | 2.1 | 1.0
Available-for-sale, gross unrealized losses | 0.0 | (0.4)
Available-for-sale, estimated fair value | 1,295.3 | 501.6
Due in one year or less |  |
Schedule of available-for-sale securities, debt maturities [Line Items] |  |
Available-for-sale, adjusted cost | 246.5 | 342.7
Available-for-sale, gross unrealized gains | 0.0 | 0.0
Available-for-sale, gross unrealized losses | 0.0 | (0.2)
Available-for-sale, estimated fair value | 246.5 | 342.5
Due after one year and through five years |  |
Schedule of available-for-sale securities, debt maturities [Line Items] |  |
Available-for-sale, adjusted cost | 1,046.0 | 157.6
Available-for-sale, gross unrealized gains | 0.0 | 0.1
Available-for-sale, gross unrealized losses | 0.0 | (0.2)
Available-for-sale, estimated fair value | 1,046.0 | 157.5
Due after five years and through ten years |  |
Schedule of available-for-sale securities, debt maturities [Line Items] |  |
Available-for-sale, adjusted cost | 0.0 | 0.0
Available-for-sale, gross unrealized gains | 0.0 | 0.0
Available-for-sale, gross unrealized losses | 0.0 | 0.0
Available-for-sale, estimated fair value | 0.0 | 0.0
Due after ten years |  |
Schedule of available-for-sale securities, debt maturities [Line Items] |  |
Available-for-sale, adjusted cost | 0.0 | 0.0
Available-for-sale, gross unrealized gains | 0.0 | 0.0
Available-for-sale, gross unrealized losses | 0.0 | 0.0
Available-for-sale, estimated fair value | 0.0 | 0.0
Total Maturities |  |
Schedule of available-for-sale securities, debt maturities [Line Items] |  |
Available-for-sale, adjusted cost | 1,292.5 | 500.3
Available-for-sale, gross unrealized gains | 0.0 | 0.1
Available-for-sale, gross unrealized losses | 0.0 | (0.4)
Available-for-sale, estimated fair value | $ 1,292.5 | $ 500.0

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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