Preferred Stocks (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Preferred stocks	$ 15,000	$ 15,000
Preferred stock redemption price upon liquidation (in dollars per share)	$ 100
Cumulative Preferred Stock, Par Value $100 Issuable in Series [Member]
Class of Stock [Line Items]
Authorized preferred cumulative shares (in shares)	500,000	500,000
Cumulative preferred shares, par value (in dollars per share)	$ 100.00	$ 100.00
Preferred stock, series 4.50% [Member]
Class of Stock [Line Items]
Preferred stocks	$ 10,000	$ 10,000
Preferred shares outstanding (in shares)	100,000	100,000
Preferred stock, $100 par value	4.50%	4.50%	4.50%
Preferred stock, dividends per share, declared	$ 4.50	$ 4.50	$ 4.50
Preferred stock, redemption per share amount	$ 105
Preferred stock, series 4.70% [Member]
Class of Stock [Line Items]
Preferred stocks	$ 5,000	$ 5,000
Preferred shares outstanding (in shares)	50,000	50,000
Preferred stock, $100 par value	4.70%	4.70%	4.70%
Preferred stock, dividends per share, declared	$ 4.70	$ 4.70	$ 4.70
Preferred stock, redemption per share amount	$ 102
Cumulative preferred stock A, without par value, issuable in series [Member]
Class of Stock [Line Items]
Authorized preferred cumulative shares (in shares)	1,000,000	1,000,000
Cumulative preference stock , without par value, issuable in series [Member]
Class of Stock [Line Items]
Authorized preferred cumulative shares (in shares)	500,000	500,000