Condensed Financial Information of Registrant (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Cash flows from operating activities:
Net income	$ 29,909	$ 28,951	$ 46,908	$ 34,915	$ 25,728	$ 41,911	$ 43,460	$ 31,823	$ 140,683	[1]	$ 142,922	[1]	$ 50,716	[1]
Adjustments to reconcile net income to net cash provided by operating activities:
Net cash provided by operating activities									270,210		256,372		237,253
Cash flows from investing activities:
Other									(2,691)		(2,541)		(2,313)
Net cash used in investing activities									(219,242)		(126,731)		(219,258)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									2,859		17,233		9,533
Net cash used in financing activities									(93,765)		(72,013)		(13,514)
Increase (decrease) in cash and cash equivalents									(43,646)		56,594		4,784
Cash and cash equivalents at beginning of year				125,029				68,435	125,029		68,435		63,651
Cash and cash equivalents at end of year	81,383				125,029				81,383		125,029		68,435
Parent Company [Member]
Cash flows from operating activities:
Net income									135,438		137,511		45,551
Adjustments to reconcile net income to net cash provided by operating activities:
Equity interest in earnings of subsidiaries									(135,438)		(137,511)		(45,551)
Increase in intercompany receivable									0		0		0
Net cash provided by operating activities									0		0		0
Cash flows from investing activities:
Other									(2,859)		(17,233)		(9,533)
Net cash used in investing activities									(2,859)		(17,233)		(9,533)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									2,859		17,233		9,533
Net cash used in financing activities									2,859		17,233		9,533
Increase (decrease) in cash and cash equivalents									0		0		0
Cash and cash equivalents at beginning of year				0				0	0		0		0
Cash and cash equivalents at end of year	$ 0				$ 0				$ 0		$ 0		$ 0

[1] Includes $2.4 million, $3.3 million and $2.5 million for 2014, 2013 and 2012, respectively, related to redeemable and mandatorily redeemable noncontrolling interests.