Summary of Selected Quarterly Financial Data (Unaudited) (Details) (USD $)	3 Months Ended										12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Summary of Selected Quarterly Financial Data (Unaudited) [Abstract]
Revenues	$ 375,401,000		$ 321,976,000	$ 333,081,000	$ 334,813,000	$ 354,589,000		$ 372,882,000	$ 371,268,000	$ 338,030,000	$ 1,365,271,000	$ 1,436,769,000	$ 1,345,923,000
Income from operations	30,526,000		25,606,000	26,306,000	31,107,000	31,320,000		31,313,000	26,092,000	23,846,000	113,545,000	112,571,000	79,794,000
Net income	16,933,000	[1]	15,552,000	16,279,000	19,602,000	13,947,000	[2]	19,452,000	14,614,000	12,993,000	68,366,000	61,006,000	67,394,000
Basic income per share	$ 0.28		$ 0.26	$ 0.27	$ 0.33	$ 0.29		$ 0.33	$ 0.25	$ 0.22	$ 1.13	$ 1.03	$ 1.14
Diluted income per share (in dollars per share)	$ 0.28		$ 0.26	$ 0.27	$ 0.33	$ 0.29		$ 0.33	$ 0.25	$ 0.22	$ 1.13	$ 1.02	$ 1.13
Debt extinguishment expense											0	10,261,000	0
Debt extinguishment net of tax												6,300,000
Write off option payment											10,000,000
Write-off of non-current asset, net of tax											6,100,000
Available-for-sale Securities, Gross Realized Gains											$ 3,700,000

[1] In December 2013, the company recorded a $10.0 million pretax charge ($6.1 million after tax) to write off a non-current asset and a $3.7 million gain (pretax and after tax) on the sale of investments.

[2] In December 2012, the company recorded a $10.3 million pretax debt extinguishment charge ($6.3 million after tax) related to a financing transaction.