Senior Notes, Convertible Notes and Trust Preferred Securities - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2007	May 31, 2014	Jun. 30, 2013	Dec. 31, 2010
Accounts, Notes, Loans and Financing Receivable [Line Items]
Debt satisfaction (gains) charges, net	$ (6,196,000)	$ 975,000	$ (25,150,000)
Convertible Debt [Member] | Six Percent Convertible Guaranteed Note [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Face amount							$ 115,000,000
Debt interest rate percentage							6.00%
Original debt, amount		$ 12,400,000	$ 3,828,000
Shares issued (in shares)		1,892,269	519,664
Cash payments		$ 672,000	$ 529,000
Debt satisfaction (gains) charges, net		436,000	$ 476,000
6.804% Trust Preferred Securities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Face amount				$ 200,000,000
Debt interest rate percentage				6.804%
Effective interest percentage	3.078%
Principal amount outstanding on Trust Preferred Securities	$ 129,120,000	129,120,000
Debt issuance costs, gross	1,924,000	2,024,000
6.804% Trust Preferred Securities [Member] | London Interbank Offered Rate (LIBOR) [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Basis spread on variable rate				1.70%
Senior Notes [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Face amount	500,000,000	500,000,000
Long-term debt	495,198,000	494,362,000
Senior Notes [Member] | Senior Notes Due 2024 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Face amount	$ 250,000,000	$ 250,000,000
Debt interest rate percentage	4.40%	4.40%			4.40%
Senior Notes [Member] | Senior Notes Due 2023 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Face amount	$ 250,000,000	$ 250,000,000
Debt interest rate percentage	4.25%	4.25%				4.25%