Debt, Cash Convertible Notes (Details)			1 Months Ended	12 Months Ended
	Oct. 01, 2013 USD ($) $ / shares	Jul. 16, 2013 USD ($) $ / shares	Oct. 31, 2016 $ / shares shares	Dec. 31, 2017 USD ($) TradingDay	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jul. 31, 2013 $ / shares
Debt Instrument [Line Items]
Interest expense related to amortization of debt discount				$ 8,001,000	$ 7,564,000	$ 7,148,000
Minimum annual principal payments and repayments of the revolving advances [Abstract]
2018				150,000,000
2019				0
2020				0
2021				0
2022				0
2023 and thereafter				0
Total				$ 150,000,000
CareFirst Convertible Note [Member]
Debt Instrument [Line Items]
Aggregate principal of convertible notes	$ 20,000,000
Proceeds from issuance of convertible debt	$ 20,000,000
Initial conversion price (in dollars per share) | $ / shares	$ 22.41		$ 22.41
Debt converted instrument shares issued (in shares) | shares			892,458
1.50% Cash Convertible Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Aggregate principal of convertible notes		$ 150,000,000
Interest rate for notes		1.50%
Cash conversion rate		51.3769
Par amount of convertible note		$ 1,000
Consecutive trading days | TradingDay				80
Initial conversion price (in dollars per share) | $ / shares		$ 19.4640
Deferred loan costs		$ 3,900,000
Fair value of cash conversion derivative		$ 36,800,000
Interest expense related to amortization of debt discount				$ 8,000,000	7,600,000
Effective interest rate				5.70%
Interest expense				$ 2,300,000	2,300,000
Debt instrument, unamortized discount				$ 4,100,000	$ 12,100,000
Warrants strike price (in dollars per share) | $ / shares							$ 25.95