Earning per Share and Diluted (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012		Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share Disclosure [Line Items]
Net income	$ 6,621	$ 20,668	$ 9,578	$ 82,472	[1]	$ 6,204	$ 7,870	$ 8,663	$ 71,412	[1]	$ 119,339	$ 94,149	$ 64,753
Weighted average shares outstanding											20,959	20,945	20,877
Effect of dilutive securities											69	48	83
Average shares outstanding including dilutive securities											21,028	20,993	20,960
Class A Common Stock
BASIC EARNINGS PER SHARE:
Basic earnings per share	$ 0.33	$ 0.99	$ 0.46	$ 3.94	[1]	$ 0.30	$ 0.38	$ 0.42	$ 3.41	[1]	$ 5.71	$ 4.50	$ 3.11
DILUTED EARNINGS PER SHARE:
Diluted earnings per share	$ 0.33	$ 0.98	$ 0.46	$ 3.92	[1]	$ 0.30	$ 0.38	$ 0.41	$ 3.40	[1]	$ 5.69	$ 4.49	$ 3.10
Class B Common Stock
BASIC EARNINGS PER SHARE:
Basic earnings per share	$ 0.21	$ 0.97	$ 0.44	$ 3.92	[1]	$ 0.28	$ 0.36	$ 0.40	$ 3.40	[1]	$ 5.55	$ 4.45	$ 3.06
DILUTED EARNINGS PER SHARE:
Diluted earnings per share	$ 0.21	$ 0.97	$ 0.44	$ 3.90	[1]	$ 0.28	$ 0.36	$ 0.40	$ 3.39	[1]	$ 5.53	$ 4.44	$ 3.04

[1] The first quarter of 2012 and 2011 was significantly impacted by the TRS operating division.