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Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Comprehensive Income Detail [Abstract]
Comprehensive income available for common stockholders	$ 156	$ 87	$ 80
Earnings per share on comprehensive income:
Basic	$ 2.12	$ 1.24	$ 1.21
Diluted	$ 2.09	$ 1.22	$ 1.19
Weighted average common shares outstanding:
Weighted Average Number of Shares Outstanding, Basic	73,438,209	69,926,933	66,013,247
Weighted Average Number of Shares Outstanding, Diluted	74,266,370	70,904,921	67,299,413

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Basic earnings per share on comprehensive income

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Diluted earnings per share on comprehensive income

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Earnings Per Share On Comprehensive Income [Abstract]

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Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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