Share-based compensation Nonqualified Stock Options (Details) - Nonqualified stock options (NQSOs) - $ / shares	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding at the beginning of the period (in shares)	14,000	55,500
Outstanding, at the beginning of the period (in dollars per share)	$ 20.49	$ 20.92
Granted (in shares)	0	0
Granted (in dollars per share)	$ 0.00	$ 0.00
Exercised (in shares)	(14,000)	(41,500)
Exercised (in dollars per share)	$ 20.49	$ 21.06
Forfeited (in shares)	0	0
Forfeited (in dollars per share)	$ 0.00	$ 0.00
Expired (in shares)	0	0
Expired (in dollars per share)	$ 0.00	$ 0.00
Outstanding at the end of the period (in shares)	0	14,000
Outstanding, at the end of the period (in dollars per share)	$ 0.00	$ 20.49
Exercisable (in shares)	0	14,000
Exercisable (in dollars per share)	$ 0.00	$ 20.49