Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 64,154	$ 56,468	$ 55,923	$ 39,553	$ 29,909	$ 28,951	$ 46,908	$ 34,915	$ 216,098	$ 140,683	$ 142,922
Adjustments to reconcile net income to net cash provided by operating activities:
Net cash provided by operating activities									317,545	270,210	256,372
Cash flows used in investing activities:
Decrease in intercompany receivable									(2,042)	(2,691)	(2,541)
Net cash used in investing activities									(203,652)	(219,242)	(126,731)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									2,683	2,859	17,233
Net cash used in financing activities									(95,273)	(93,765)	(72,013)
Increase (decrease) in cash and cash equivalents									17,505	(43,646)	56,594
Cash and cash equivalents at beginning of year				81,383				125,029	81,383	125,029	68,435
Cash and cash equivalents at end of year	98,888				81,383				98,888	81,383	125,029
Parent Company [Member]
Cash flows from operating activities:
Net income									211,049	135,438	137,511
Adjustments to reconcile net income to net cash provided by operating activities:
Equity interest in earnings of subsidiaries									(211,049)	(135,438)	(137,511)
Net cash provided by operating activities									0	0	0
Cash flows used in investing activities:
Decrease in intercompany receivable									(2,683)	(2,859)	(17,233)
Net cash used in investing activities									(2,683)	(2,859)	(17,233)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									2,683	2,859	17,233
Net cash used in financing activities									2,683	2,859	17,233
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