Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 42,476	$ 60,779	$ 66,426	$ 47,722	$ 64,154	$ 56,468	$ 55,923	$ 39,553	$ 217,403	$ 216,098	$ 140,683
Adjustments to reconcile net income to net cash provided by operating activities:
Net cash provided by operating activities									389,831	317,545	270,210
Cash flows used in investing activities:
Decrease in intercompany receivable									(9,351)	(2,042)	(2,691)
Net cash used in investing activities									(828,241)	(203,652)	(219,242)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									3,965	2,683	2,859
Net cash provided by (used in) financing activities									421,316	(95,273)	(93,765)
(Decrease) increase in cash and cash equivalents									(17,479)	17,505	(43,646)
Cash and cash equivalents at beginning of year				98,888				81,383	98,888	81,383	125,029
Cash and cash equivalents at end of year	81,409				98,888				81,409	98,888	81,383
Parent Company
Cash flows from operating activities:
Net income									209,196	211,049	135,438
Adjustments to reconcile net income to net cash provided by operating activities:
Equity interest in earnings of subsidiaries									(209,196)	(211,049)	(135,438)
Net cash provided by operating activities									0	0	0
Cash flows used in investing activities:
Decrease in intercompany receivable									(3,965)	(2,683)	(2,859)
Net cash used in investing activities									(3,965)	(2,683)	(2,859)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock incentive plans									3,965	2,683	2,859
Net cash provided by (used in) financing activities									3,965	2,683	2,859
(Decrease) increase 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