CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Millions	Total	Total Common Shareholders' Equity	Common stock	Additional paid in capital	Retained earnings	Preferred stock
Balance at Dec. 31, 2017	$ 3,414.3	$ 3,383.9	$ 332.9	$ 802.7	$ 2,248.3	$ 30.4
Condensed consolidated statements of equity
Net income attributed to common shareholder	105.8	105.8	0.0	0.0	105.8	0.0
Payment of dividends to parent	(60.0)	(60.0)	0.0	0.0	(60.0)	0.0
Equity contribution from parent	28.0	28.0	0.0	28.0	0.0	0.0
Stock-based compensation and other	0.2	0.2	0.0	0.2	0.0	0.0
Balance at Mar. 31, 2018	3,488.3	3,457.9	332.9	830.9	2,294.1	30.4
Balance at Dec. 31, 2017	3,414.3	3,383.9	332.9	802.7	2,248.3	30.4
Condensed consolidated statements of equity
Net income attributed to common shareholder	301.8
Equity contribution from parent	28.0
Balance at Sep. 30, 2018	3,544.7	3,514.3	332.9	831.2	2,350.2	30.4
Balance at Mar. 31, 2018	3,488.3	3,457.9	332.9	830.9	2,294.1	30.4
Condensed consolidated statements of equity
Net income attributed to common shareholder	92.8	92.8	0.0	0.0	92.8	0.0
Payment of dividends to parent	(60.0)	(60.0)	0.0	0.0	(60.0)	0.0
Stock-based compensation and other	0.3	0.3	0.0	0.3	0.0	0.0
Balance at Jun. 30, 2018	3,521.4	3,491.0	332.9	831.2	2,326.9	30.4
Condensed consolidated statements of equity
Net income attributed to common shareholder	103.2	103.2	0.0	0.0	103.2	0.0
Payment of dividends to parent	(80.0)	(80.0)	0.0	0.0	(80.0)	0.0
Stock-based compensation and other	0.1	0.1	0.0	0.0	0.1	0.0
Balance at Sep. 30, 2018	3,544.7	3,514.3	332.9	831.2	2,350.2	30.4
Balance at Dec. 31, 2018	3,491.2	3,460.8	332.9	831.3	2,296.6	30.4
Condensed consolidated statements of equity
Net income attributed to common shareholder	114.7	114.7	0.0	0.0	114.7	0.0
Payment of dividends to parent	(150.0)	(150.0)	0.0	0.0	(150.0)	0.0
Stock-based compensation and other	0.2	0.2	0.0	0.2	0.0	0.0
Balance at Mar. 31, 2019	3,456.1	3,425.7	332.9	831.5	2,261.3	30.4
Balance at Dec. 31, 2018	3,491.2	3,460.8	332.9	831.3	2,296.6	30.4
Condensed consolidated statements of equity
Net income attributed to common shareholder	300.2
Equity contribution from parent	105.0
Balance at Sep. 30, 2019	3,619.5	3,589.1	332.9	929.4	2,326.8	30.4
Balance at Mar. 31, 2019	3,456.1	3,425.7	332.9	831.5	2,261.3	30.4
Condensed consolidated statements of equity
Net income attributed to common shareholder	84.9	84.9	0.0	0.0	84.9	0.0
Payment of dividends to parent	(60.0)	(60.0)	0.0	0.0	(60.0)	0.0
Equity contribution from parent	105.0	105.0	0.0	105.0	0.0	0.0
Transfer of net assets to UMERC	(7.3)	(7.3)	0.0	(7.3)	0.0	0.0
Balance at Jun. 30, 2019	3,578.7	3,548.3	332.9	929.2	2,286.2	30.4
Condensed consolidated statements of equity
Net income attributed to common shareholder	100.6	100.6	0.0	0.0	100.6	0.0
Payment of dividends to parent	(60.0)	(60.0)	0.0	0.0	(60.0)	0.0
Stock-based compensation and other	0.2	0.2	0.0	0.2	0.0	0.0
Balance at Sep. 30, 2019	$ 3,619.5	$ 3,589.1	$ 332.9	$ 929.4	$ 2,326.8	$ 30.4

Amount of other increase (decrease) in additional paid in capital (APIC).

No definition available.

Name:	us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_DividendsCommonStockCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_IncreaseDecreaseInStockholdersEquityRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e1377-109256

+ Details

Name:	us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from parent as a source of financing that is recorded as additional paid in capital.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromContributionsFromParent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(31))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_StockholdersEquity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents movements included in the statement of changes in stockholders' equity which are not separately disclosed or provided for elsewhere in the taxonomy.

+ References

No definition available.

+ Details

Name:	us-gaap_StockholdersEquityOther
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration