Preferred and Common Stockholder's Equity - Schedule of Stock Repurchases (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Dec. 31, 2018	Dec. 31, 2018	Jun. 30, 2019
Stockholders Equity [Line Items]
Shares purchased and retired			1.6
Common stock and additional paid-in-capital reductions	$ 16.9	$ 16.9
Charges to retained earnings	$ 33.1	$ 33.1
Common Stock [Member]
Stockholders Equity [Line Items]
Shares purchased and retired	1.6	1.6

Number of shares that have been repurchased and retired during the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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Period Type:	duration