Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Class of Stock
Preferred stock	$ 30.4	$ 30.4
Six per cent. preferred stock
Class of Stock
Preferred stock, par or stated value per share	$ 100	$ 100
Preferred stock, dividend rate, percentage	6.00%	6.00%
Preferred stock, shares authorized	45,000	45,000
Preferred stock, shares outstanding	44,498	44,498
Preferred stock, redemption price per share	$ 0	$ 0
Preferred stock	$ 4.4	$ 4.4
Serial preferred stock, $100 par value; authorized 2,286,500 shares
Class of Stock
Preferred stock, par or stated value per share	$ 100	$ 100
Preferred stock, shares authorized	2,286,500	2,286,500
Serial preferred stock, 3.60% series redeemable
Class of Stock
Preferred stock, par or stated value per share	$ 100	$ 100
Preferred stock, dividend rate, percentage	3.60%	3.60%
Preferred stock, shares outstanding	260,000	260,000
Preferred stock, redemption price per share	$ 101	$ 101
Preferred stock	$ 26.0	$ 26.0
Serial preferred stock, $25 par value; authorized 5,000,000 shares; none outstanding
Class of Stock
Preferred stock, par or stated value per share	$ 25	$ 25
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares outstanding	0	0
Preferred stock, redemption price per share	$ 0	$ 0
Preferred stock	$ 0.0	$ 0.0