Subsequent Events (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended							9 Months Ended		183 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Feb. 05, 2013
Subsequent Event [Line Items]
Average price per share of repurchased shares (in dollars per share)	$ 77.67				$ 72.40			$ 78.43	$ 57.97	$ 43.53
Shares available to be repurchased (in shares)											500,000
Shares authorized for issuance (in shares)	1,500,000							1,500,000
Dividends declared (in dollars per share)	$ 0.06				$ 0.06			$ 0.18	$ 0.16
Dividends payable date	Sep. 30, 2014	Jun. 24, 2014	Mar. 25, 2014		Sep. 24, 2013	Jun. 25, 2013	Mar. 26, 2013
Dividends record date	Sep. 16, 2014	Jun. 10, 2014	Mar. 11, 2014		Sep. 10, 2013	Jun. 11, 2013	Mar. 12, 2013
Expected expense related to Shareholder-designated charitable contribution program				$ 5,300				$ 134	$ 5,313
Percentage stock based compensation expense recognize over three years vesting period (in hundredths)	30.00%
Stock based compensation expense recognize over five years vesting period	70.00%
Vesting period for recognition of 30 percent of stock based compensation expense	3 years
Vesting period for recognition of 70 percent of stock based compensation expense	5 years
Restricted Stock [Member]
Subsequent Event [Line Items]
Percentage stock based compensation expense recognize over three years vesting period (in hundredths)	30.00%
Stock based compensation expense recognize over five years vesting period	70.00%
Vesting period for recognition of 30 percent of stock based compensation expense	3 years
Vesting period for recognition of 70 percent of stock based compensation expense	5 years
Subsequent Event [Member]
Subsequent Event [Line Items]
Stock repurchased (in shares)	59,539
Average price per share of repurchased shares (in dollars per share)	$ 77.42
Special dividend declared (in dollars per share)	$ 0.25
Quarterly Dividend [Member]
Subsequent Event [Line Items]
Dividends declared (in dollars per share)	$ 0.06	$ 0.06	$ 0.06		$ 0.06	$ 0.05	$ 0.05
Quarterly Dividend [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends declared (in dollars per share)	$ 0.07
Dividends declared date	Nov. 06, 2014
Dividends payable date	Dec. 30, 2014
Dividends record date	Dec. 16, 2014