Quarterly Financial Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 119.8	$ 76.6	$ 73.3	$ 156.1	$ 109.8	$ 72.5	$ 66.4	$ 118.2	$ 425.8	$ 366.9	$ 353.1
Operating Income	20.1us-gaap_OperatingIncomeLoss	7.4us-gaap_OperatingIncomeLoss	7.1us-gaap_OperatingIncomeLoss	25.4us-gaap_OperatingIncomeLoss	21.5us-gaap_OperatingIncomeLoss	5.6us-gaap_OperatingIncomeLoss	4.5us-gaap_OperatingIncomeLoss	21.9us-gaap_OperatingIncomeLoss	60.0us-gaap_OperatingIncomeLoss	53.5us-gaap_OperatingIncomeLoss	47.5us-gaap_OperatingIncomeLoss
Net Income (Loss) Applicable to Common	$ 9.4	$ 1.6	$ 1.1	$ 12.6	$ 10.3	$ 0.6	$ (0.1)	$ 10.8	$ 24.7	$ 21.6	$ 18.1
Per Share Data:
Earnings Per Common Share	$ 0.69us-gaap_EarningsPerShareBasicAndDiluted	$ 0.11us-gaap_EarningsPerShareBasicAndDiluted	$ 0.08us-gaap_EarningsPerShareBasicAndDiluted	$ 0.91us-gaap_EarningsPerShareBasicAndDiluted	$ 0.75us-gaap_EarningsPerShareBasicAndDiluted	$ 0.04us-gaap_EarningsPerShareBasicAndDiluted	$ (0.01)us-gaap_EarningsPerShareBasicAndDiluted	$ 0.79us-gaap_EarningsPerShareBasicAndDiluted	$ 1.79us-gaap_EarningsPerShareBasicAndDiluted	$ 1.57us-gaap_EarningsPerShareBasicAndDiluted	$ 1.43us-gaap_EarningsPerShareBasicAndDiluted
Dividends Paid Per Common Share	$ 0.345	$ 0.345	$ 0.345	$ 0.345	$ 0.345	$ 0.345	$ 0.345	$ 0.345	$ 1.38	$ 1.38	$ 1.38

Aggregate dividends paid during the period for each share of common stock outstanding.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-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

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

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