Equity, Voting Rights and Stock Award and Incentive Plan (Details) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) VoteperShare Plan $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2012 USD ($)
Stock Award and Incentive Plan [Abstract]
Number of incentive plans | Plan		2
RSAs, weighted average grant date fair value [Abstract]
Stock compensation expense recognized due to acceleration of lapsing of restrictions on RSAs	$ 3,500
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Compensation cost related to non-vested restricted stock awards and options not yet recognized	10,800	$ 10,800
Projected compensation cost not yet recognized [Abstract]
2016	3,569	3,569
2017	2,492	2,492
2018	1,685	1,685
2019	1,348	1,348
2020	737	737
2021	497	497
2022	300	300
2023	127	127
2024	$ 19	19
Stock based compensation expense		9,868	$ 5,278	$ 1,562
Tax benefit from compensation expense		3,700	2,000	600
Proceeds from exercise of stock options		1,167	1,637	76
Tax benefit from exercise of stock options		$ 102	$ 349	$ 16
Class A [Member]
Voting Rights [Abstract]
Number of votes per share | VoteperShare		1
Class A [Member] | Maximum [Member]
Stock Award and Incentive Plan [Abstract]
Number of shares reserved for issuance under each plan (in shares) | shares	3,500,000	3,500,000
Class B [Member]
Voting Rights [Abstract]
Number of votes per share | VoteperShare		10
Stock Options [Member]
Options, shares [Roll Forward]
Outstanding, beginning of period (in shares) | shares		26,000	66,000
Granted (in shares) | shares		0	0
Forfeited (in shares) | shares		0	0
Exercised (in shares) | shares		(26,000)	(40,000)
Outstanding, end of period (in shares) | shares	0	0	26,000	66,000
Shares available for future issuance, end of period (in shares) | shares	1,856,925	1,856,925
Options, weighted average exercise price [Roll Forward]
Outstanding, beginning of period (in dollars per share) | $ / shares		$ 44.89	$ 42.49
Granted (in dollars per share) | $ / shares		0	0
Forfeited (in dollars per share) | $ / shares		0	0
Exercised / Vested (in dollars per share) | $ / shares		39.55	40.94
Outstanding, end of period (in dollars per share) | $ / shares	$ 0	$ 0	$ 44.89	$ 42.49
RSAs, weighted average grant date fair value [Abstract]
Options exercisable (in shares) | shares			23,500
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares			$ 44.80
Stock Options [Member] | Maximum [Member]
Stock Award and Incentive Plan [Abstract]
Term of nonqualified stock options		10 years
Stock Options [Member] | Vesting in Three Years from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		75.00%
Award vesting period		3 years
Restricted Stock Awards [Member]
RSAs, shares [Roll Forward]
Outstanding, beginning of period (in shares) | shares		710,750	566,950
Granted (in shares) | shares		0	158,600	576,950
Forfeited (in shares) | shares		(27,000)	(14,800)
Vested (in shares) | shares		(130,650)	0
Outstanding, end of period (in shares) | shares	553,100	553,100	710,750	566,950
RSAs, weighted average grant date fair value [Abstract]
Outstanding, beginning of period (in dollars per share) | $ / shares		$ 67.45	$ 63.93
Granted (in dollars per share) | $ / shares		0	80.23	$ 63.82
Forfeited (in dollars per share) | $ / shares		69.50	69.38
Vested (in dollars per share) | $ / shares		81.55	0
Outstanding, end of period (in dollars per share) | $ / shares	$ 64.02	$ 64.02	$ 67.45	$ 63.93
Stock compensation expense recognized due to acceleration of lapsing of restrictions on RSAs					$ 3,500
Restricted Stock Awards [Member] | Vesting in Three Years from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		30.00%
Award vesting period		3 years
Restricted Stock Awards [Member] | Vesting in Five Years from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		70.00%
Award vesting period		5 years
Restricted Stock Awards [Member] | Vesting in Year Four from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		10.00%
Restricted Stock Awards [Member] | Vesting in Year Five from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		10.00%
Restricted Stock Awards [Member] | Vesting in Year Six from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		10.00%
Restricted Stock Awards [Member] | Vesting in Year Seven from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		10.00%
Restricted Stock Awards [Member] | Vesting in Year Eight from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		10.00%
Restricted Stock Awards [Member] | Vesting in Year Nine from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		10.00%
Restricted Stock Awards [Member] | Vesting in Year Ten from Date of Grant [Member]
RSAs, weighted average grant date fair value [Abstract]
Award vesting percentage		10.00%