Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions													12 Months Ended
	Dec. 21, 2017	Sep. 21, 2017	Jun. 30, 2017	Mar. 23, 2017	Dec. 22, 2016	Sep. 22, 2016	Jun. 23, 2016	Mar. 24, 2016	Dec. 17, 2015	Sep. 24, 2015	Jun. 18, 2015	Mar. 26, 2015	Dec. 31, 2017	Apr. 30, 2017	Feb. 25, 2016	May 09, 2012
Shareholders equity
Number of shares of common stock for each warrant unit																1
Number of common shares that can be acquired upon exercise of each warrant unit																0.5
Exercise price of outstanding warrants (in dollars per share)													$ 16.70
Number of warrant shares purchasable (in shares)													1,232,916
Shares authorized to be repurchased													2,100,000		2,050,000
Dividends declared per Share of Common Stock (in dollars per share)	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.45	$ 0.58	$ 0.60	$ 0.64	$ 0.67
Dividends paid per Share of Common Stock (in dollars per share)	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.45	$ 0.58	$ 0.6	$ 0.64	$ 0.67
Institutional investors
Shareholders equity
Number of warrant units authorized to be sold																2,231,787
Common Stock
Shareholders equity
Treasury stock (in shares)													125,722
Common Stock
Shareholders equity
Equity distribution agreement, maximum value														$ 100.0
Treasury stock (in shares)													125,722