Senior Notes (Narrative) (Details) - Senior Notes 4.868% [Member] - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt maturity term	20 years
Debt instrument, stated interest rate	4.868%
Maturity date	Jun. 01, 2044
Senior Notes [Member]
Debt Instrument [Line Items]
Date Issued	Jun. 05, 2014
Principal Amounts	$ 400,000,000
Debt maturity term	30 years
Debt instrument, stated interest rate	4.868%	4.868%
Maturity date	Jun. 01, 2044	Jun. 01, 2044

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

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- Definition

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

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- Definition

Date the debt instrument was issued, in CCYY-MM-DD format.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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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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