Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common stock transactions:
Stock options exercised (in shares)	285,887
Cash dividends declared:
Common (in dollars per share)	$ 0.96	$ 0.88	$ 0.8
Common stock
Common stock transactions:
Common shares purchased and retired	33,063	1,015,389	793,077
Common shares issued	11,267,676	16,347	21,414
Non-vested common shares issued	140,246	190,239	169,577
Non-vested common shares forfeited	53,571	29,844	19,184
Stock options exercised (in shares)	218,095	336,598	261,080
Shares tendered in payment of option price and income tax withholding amounts	67,792	104,643	89,358