Stockholders' Equity (Details)	3 Months Ended							9 Months Ended		12 Months Ended	201 Months Ended
	Sep. 30, 2015 USD ($) VoteperShare IncentivePlan $ / shares shares	Jun. 30, 2015 $ / shares	Mar. 31, 2015 $ / shares	Dec. 31, 2014 USD ($) shares	Sep. 30, 2014 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) $ / shares	Mar. 31, 2014 USD ($) $ / shares	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2014 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Aug. 04, 2015 shares
Stockholders' Equity [Abstract]
Shares outstanding (in shares) | shares	25,500,000			25,900,000	25,900,000			25,500,000	25,900,000	25,900,000	25,500,000
Dividends Payable [Line Items]
Dividends declared (in dollars per share) | $ / shares	$ 0.07				$ 0.06			$ 0.21	$ 0.18
Dividend payment date	Sep. 29, 2015	Jun. 30, 2015	Mar. 31, 2015		Sep. 30, 2014	Jun. 24, 2014	Mar. 25, 2014
Dividend record date	Sep. 15, 2015	Jun. 16, 2015	Mar. 17, 2015		Sep. 16, 2014	Jun. 10, 2014	Mar. 11, 2014
Stock Award and Incentive Plan [Abstract]
Number of incentive plans | IncentivePlan	2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for issuance under each plan, maximum (in shares) | shares	3,500,000							3,500,000			3,500,000
Term of nonqualified stock options, maximum (in years)	10 years
RSA shares outstanding (in shares) | shares	688,550			710,750	639,750			688,550	639,750	710,750	688,550
Average weighted grant price (in dollars per share) | $ / shares								$ 67.34	$ 65.12	$ 67.45
Percentage stock based compensation expense recognize over three years vesting period (in hundredths)	30.00%
Percentage stock based compensation expense recognize over five years vesting period (in hundredths)	70.00%
Vesting period for recognition of 30 percent of stock based compensation expense (in years)	3 years
Vesting period for recognition of 70 percent of stock based compensation expense (in years)	5 years
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Actual stock based compensation expense	$ 2,275,000				$ 1,785,000			$ 6,819,000	$ 5,182,000
Projected stock based compensation expense 2014										$ 7,199,000
Projected stock based compensation expense 2015	14,800,000
Projected stock based compensation expense 2016	5,839,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Three	4,182,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Ten	33,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Six	1,160,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Seven	778,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Nine	201,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Four	2,800,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Five	2,272,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Eight	472,000
Compensation cost related to non-vested restricted stock awards and options not yet recognized	$ 25,700,000							$ 25,700,000			$ 25,700,000
Stock options exercised (in shares) | shares	0				0			26,000	40,000
Proceeds from exercise of stock options	$ 0				$ 0			$ 1,169,000	$ 1,637,000
Tax benefit from exercise of stock options	$ 0				$ 0			$ 102,000	$ 349,000
Stock Repurchase Program [Abstract]
Incremental Class A shares authorized to buyback (in shares) | shares												500,000
Shares repurchased (in shares) | shares	172,007				94,942			321,488	319,662		9,447,513
Average price per share of repurchased shares (in dollars per share) | $ / shares	$ 61.11				$ 77.67			$ 66.96	$ 78.43		$ 44.70
Share available under program to repurchase (in shares) | shares	687,295							687,295			687,295
Quarterly Dividend [Member]
Dividends Payable [Line Items]
Dividends declared (in dollars per share) | $ / shares	$ 0.07	$ 0.07	$ 0.07		$ 0.06	$ 0.06	$ 0.06
Q1 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Actual stock based compensation expense							$ 1,700,000
Projected stock based compensation expense 2015	$ 2,277,000
Projected stock based compensation expense 2016	1,681,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Three	1,143,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Ten	12,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Six	350,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Seven	227,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Nine	76,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Four	770,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Five	612,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Eight	146,000
Q2 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Projected stock based compensation expense 2014						$ 1,697,000
Projected stock based compensation expense 2015	2,267,000
Projected stock based compensation expense 2016	1,681,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Three	1,143,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Ten	12,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Six	319,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Seven	227,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Nine	76,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Four	748,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Five	612,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Eight	146,000
Q3 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Projected stock based compensation expense 2014					$ 1,785,000
Projected stock based compensation expense 2015	2,275,000
Projected stock based compensation expense 2016	1,327,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Three	1,013,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Ten	9,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Six	264,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Seven	179,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Nine	38,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Four	669,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Five	547,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Eight	104,000
Q4 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Projected stock based compensation expense 2014				$ 2,017,000
Projected stock based compensation expense 2015	7,981,000
Projected stock based compensation expense 2016	1,150,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Three	883,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Ten	0
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Six	227,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Seven	145,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Nine	11,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Four	613,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Five	501,000
Employee Service Share Based Compensation Nonvested Awards Projected Compensation Cost Year Eight	$ 76,000
Class A [Member]
Stockholders' Equity [Abstract]
Shares outstanding (in shares) | shares	6,340,992			6,616,212	6,599,982			6,340,992	6,599,982	6,616,212	6,340,992
Voting Rights [Abstract]
Number of votes per share | VoteperShare	1
Class B [Member]
Stockholders' Equity [Abstract]
Shares outstanding (in shares) | shares	19,196,792			19,239,260	19,279,260			19,196,792	19,279,260	19,239,260	19,196,792
Voting Rights [Abstract]
Number of votes per share | VoteperShare	10
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage stock based compensation expense recognize over three years vesting period (in hundredths)	30.00%
Percentage stock based compensation expense recognize over five years vesting period (in hundredths)	70.00%
Vesting period for recognition of 30 percent of stock based compensation expense (in years)	3 years
Vesting period for recognition of 70 percent of stock based compensation expense (in years)	5 years
Restricted Stock [Member] | Award Made on December 23, 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSAs granted (in shares) | shares								73,000
Restricted stock award shares approved grant date fair value (in dollars per share) | $ / shares								$ 87.99
Restricted Stock [Member] | Award Made on September 15, 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSAs granted (in shares) | shares								83,500
Restricted stock award shares approved grant date fair value (in dollars per share) | $ / shares								$ 73.41
Restricted Stock [Member] | Award Made on January 9, 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSAs granted (in shares) | shares								2,100
Restricted stock award shares approved grant date fair value (in dollars per share) | $ / shares								$ 81.99
Restricted Stock [Member] | Award Made in August 2013 and September 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage stock based compensation expense recognize over three years vesting period (in hundredths)								30.00%
Percentage stock based compensation expense recognize each year over years four through ten vesting period (in hundredths)								10.00%
Vesting period for recognition of 30 percent of stock based compensation expense (in years)								3 years