Debt - Senior Unsecured Notes Exchangeable (Details)	1 Months Ended	12 Months Ended
	Aug. 31, 2014 USD ($) d $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Senior unsecured notes, net		$ 707,895,000	$ 590,388,000
Interest expense		68,473,000	70,595,000	$ 65,743,000
Amortization expense of deferred financing costs		$ 4,700,000	5,000,000	6,100,000
Convertible debt [Member] | Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Face amount	$ 250,000,000
Stated interest rate	2.625%	2.625%
Conversion price (USD per share) | $ / shares	$ 19.45	$ 19.32
Debt, conversion ratio	0.0514059	0.0517605
Percentage of principal redeemed	100.00%
Effective rate	3.80%	3.93%
Senior unsecured notes, net	$ 236,600,000
Embedded conversion option		$ 13,400,000	13,400,000
Remaining discount amortization period	5 years
Unamortized discount		4,300,000	7,000,000
Underwriting expense	$ 3,100,000
Deferred finance costs	$ 2,900,000
Embedded conversion feature, reduction		200,000	200,000
Interest on note		9,900,000	9,900,000	9,900,000
Interest expense		6,600,000	6,600,000	6,600,000
Accretion of debt discount		2,700,000	2,700,000	2,700,000
Amortization expense of deferred financing costs		$ 600,000	$ 600,000	$ 600,000
Covenant scenario one [Member] | Convertible debt [Member] | Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Threshold percentage of stock price trigger	130.00%
Threshold trading days | d	20
Threshold consecutive trading days | d	30
Covenant scenario two [Member] | Convertible debt [Member] | Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Threshold percentage of stock price trigger	98.00%
Threshold consecutive trading days | d	5
Threshold consecutive business days	5 days