Redeemable Preferred Stock (Details) - Public Preferred Stock [Member] $ / shares in Units, $ in Millions		3 Months Ended		12 Months Ended
	Nov. 30, 1998 shares	Mar. 31, 2019 USD ($) Tranche $ / shares shares	Jun. 30, 2006 USD ($)	Dec. 31, 1991 $ / shares shares	Dec. 31, 2018 USD ($)	Dec. 31, 1990 shares
Preferred stock [Abstract]
Preferred stock authorized (in shares)		6,000,000
Preferred stock par value (in dollar per share) | $ / shares		$ 0.01
Preferred stock dividend rate per annum		12.00%		6.00%
Dividends Payable | $		$ 104.5			$ 103.5
Preferred stock issued and outstanding (in shares)		3,185,586
Preferred stock issued (in shares)						2,858,723
12% Cumulative Exchangeable Redeemable Preferred Stock [Abstract]
Adjusted accrued accretion of public preferred stock | $			$ 1.5
Number of shares declared as dividend (in shares)				736,863
Number of annual tranches during the period | Tranche		5
Period during which redeemable preferred stock not callable		12 months
Preferred stock dividend rate per annum (in dollars per share) | $ / shares		$ 1.20		$ 0.60
Preferred stock, liquidation preference (in dollars per share) | $ / shares		$ 10
Dividends on preferred stock | $		$ 1.0
Redemption of public preferred stock (in shares)	410,000