Condensed Financial Information of Registrant (Details 2) (USD $)	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
Condensed Financial Statements Captions [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 125,000,000				$ 125,000,000				$ 125,000,000	$ 125,000,000	$ 100,000,000
Cash flows from operating activities:
Net income	(57,025,000)	35,642,000	35,778,000	36,321,000	(2,060,000)	31,359,000	40,065,000	30,496,000	50,716,000	99,860,000	114,158,000
Adjustments to reconcile net income to net cash provided by operating activities:
Net cash provided by operating activities									237,253,000	191,051,000	168,073,000
Cash flows from investing activities:
Other									(2,313,000)	(596,000)	(22,000)
Net cash used in investing activities									(219,258,000)	(271,310,000)	(150,174,000)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									9,533,000	3,999,000	5,510,000
Net cash provided by (used in) financing activities									(13,514,000)	47,004,000	(66,142,000)
(Decrease) increase in cash and cash equivalents									4,784,000	(33,475,000)	(48,055,000)
Cash and cash equivalents at beginning of year				63,651,000				97,126,000	63,651,000	97,126,000	145,181,000
Cash and cash equivalents at end of year	68,435,000				63,651,000				68,435,000	63,651,000	97,126,000
Parent [Member]
Cash flows from operating activities:
Net income									45,551,000	95,405,000	110,243,000
Adjustments to reconcile net income to net cash provided by operating activities:
Equity interest in earnings of subsidiaries									(45,551,000)	(95,405,000)	(110,243,000)
Increase in intercompany receivable									0	3,999,000	5,510,000
Net cash provided by operating activities									0	(3,999,000)	(5,510,000)
Cash flows from investing activities:
Other								0	(9,533,000)		0
Net cash used in investing activities									(9,533,000)	0	0
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									9,533,000	3,999,000	5,510,000
Net cash provided by (used in) financing activities									9,533,000	3,999,000	5,510,000
(Decrease) increase in cash and cash equivalents									0	0	0
Cash and cash equivalents at beginning of year				0				0	0	0
Cash and cash equivalents at end of year	$ 0				$ 0				$ 0	$ 0	$ 0