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Derivatives - Derivatives Associated With Cash Flow Hedges (Details) - Cash Flow Hedge - Hedging Instruments And Hedged Items - USD ($) | Dec. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2017 | Mar. 31, 2016
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Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Notional |  |  | $ 650,000,000 |
Variability in cash flows related to debt instruments (primarily loans) |  |  |  | $ 250,000,000
Interest Rate Contract |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Variability in cash flows related to debt instruments (primarily loans) | $ 900,000,000 | $ 900,000,000 |  |
Interest Rate Swap |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Notional | 900,000,000 | 900,000,000 |  |
Derivative asset | 888,000 | $ 942,000 |  |
Derivative liability | $ 5,000 |  |  |

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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