Shareholders' Equity - Additional Information (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Employee Stock Purchase Plan (ESPP): Maximum annual contribution per employee	10.00%
Employee Stock Purchase Plan (ESPP), Shares in ESPP	107,000
Employee Stock Purchase Plan (ESPP), Shares purchased	29,000	44,000	37,000
Employee Stock Purchase Plan (ESPP), Weighted average purchase price	$ 5.29	$ 4.91	$ 7.38
Remaining shares from the trusts recorded as treasury stock	11,754,147	11,077,779
Preferred stock, shares authorized	2,500,000	2,500,000
Preferred stock, par or stated value per share	$ 0.01	$ 0.01
Preferred stock, shares outstanding	0	0
Stock Employee Compensation Trust (SECT) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares from the trusts recorded as treasury stock		2,700,000
Shares released from employee stock trust	503,000	98,000
Shares purchased by employee stock trust	0	0	0
Stock trusts, shares			3,200,000
Omnibus Stock Trust (OST) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares from the trusts recorded as treasury stock		59,000
Shares released from employee stock trust	0	0	0
Shares purchased by employee stock trust	0	0	0
Stock trusts, shares			59,000