Stock-Based Compensation Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified		9 Months Ended		12 Months Ended		9 Months Ended	12 Months Ended		9 Months Ended	12 Months Ended		9 Months Ended
	May 24, 2007 The 2007 plan	Sep. 30, 2013 The 2007 plan Restricted stock	Sep. 30, 2012 The 2007 plan Restricted stock	Dec. 31, 2012 The 2007 plan Restricted stock	Dec. 31, 2011 The 2007 plan Restricted stock	Sep. 30, 2013 The 2007 plan Restricted stock vesting over 4 years	Dec. 31, 2012 The 2007 plan Restricted stock vesting over 4 years	Dec. 31, 2011 The 2007 plan Restricted stock vesting over 4 years	Sep. 30, 2013 The 2007 plan Restricted stock vesting on the first anniversary date from date of issuance	Dec. 31, 2012 The 2007 plan Restricted stock vesting on the first anniversary date from date of issuance	Dec. 31, 2011 The 2007 plan Restricted stock vesting on the first anniversary date from date of issuance	Sep. 30, 2013 1996 Plan	Sep. 30, 2012 1996 Plan
Stock-based compensation plans
Number of shares authorized	2,000,000
Number of shares awarded, net of cancellations		1,781,858
Number Outstanding
Balance at the beginning of the period (in shares)		682,654	651,294	651,294	575,791
Shares granted						130,000	283,000	324,924	21,408	28,040	22,100
Shares cancelled		(28,198)		(8,988)	(27,477)
Shares vested		(179,616)		(270,692)	(244,044)
Balance at the end of the period (in shares)		626,248		682,654	651,294
Stock compensation expense (in dollars)		$ 2.5	$ 2.3
Remaining average vesting period for recognition of unrecognized compensation expense		2 years 3 months	2 years 3 months 18 days
Unrecognized compensation expense (in dollars)		6.7	6.5
Intrinsic value of unvested awards (in dollars)		9.9	7.8
Stock options exercised (in shares)												44,991	232,314
Options canceled (in shares)												6,500
Stock options outstanding (in shares)												85,437	211,267
Intrinsic value of outstanding stock options (in dollars)												$ 0.5	$ 0.9