Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

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

Equity - Common Stock (Details) - USD ($)	6 Months Ended
Equity - Common Stock (Details) - USD ($)	Jun. 30, 2018	Jun. 30, 2017	Mar. 20, 2018	Feb. 25, 2018
Equity, Class of Treasury Stock [Line Items]
Authorized amount of stock to repurchase			$ 250,000,000.0	$ 100,000,000
Stock repurchased during period, value	$ 148,700,000	$ 1,600,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares retired due to common stock repurchase program (in shares)	8,286,841	77,155