Condensed Financial Information of Registrant (Details 2) (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	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	$ 100,000,000
Cash flows from operating activities:
Net income	25,728,000	41,911,000	43,460,000	31,823,000	(57,025,000)	35,642,000	35,778,000	36,321,000	142,922,000	50,716,000	99,860,000
Adjustments to reconcile net income to net cash provided by operating activities:
Net cash provided by operating activities									256,372,000	237,253,000	191,051,000
Cash flows from investing activities:
Other									(2,541,000)	(2,313,000)	(596,000)
Net cash used in investing activities									(126,731,000)	(219,258,000)	(271,310,000)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									17,233,000	9,533,000	3,999,000
Net cash provided by (used in) financing activities									(72,013,000)	(13,514,000)	47,004,000
(Decrease) increase in cash and cash equivalents									56,594,000	4,784,000	(33,475,000)
Cash and cash equivalents at beginning of year				68,435,000				63,651,000	68,435,000	63,651,000	97,126,000
Cash and cash equivalents at end of year	125,029,000				68,435,000				125,029,000	68,435,000	63,651,000
Parent [Member]
Cash flows from operating activities:
Net income									137,511,000	45,551,000	95,405,000
Adjustments to reconcile net income to net cash provided by operating activities:
Equity interest in earnings of subsidiaries									(137,511,000)	(45,551,000)	(95,405,000)
Increase in intercompany receivable									0	0	3,999,000
Net cash provided by operating activities									0	0	(3,999,000)
Cash flows from investing activities:
Other								9,533,000	(17,233,000)		0
Net cash used in investing activities									(17,233,000)	(9,533,000)	0
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									17,233,000	9,533,000	3,999,000
Net cash provided by (used in) financing activities									17,233,000	9,533,000	3,999,000
(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