Employee Benefit Plans - Additional Information (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Automatic enrollment feature pre-tax	3.00%
Maximum employer contribution	50.00%
Maximum employee contributions percentage to receive 50% matching	6.00%
Employer match, percent	3.00%
Contributions vested with the employees	2 years
Contributions to plans	$ 5	$ 5	$ 3
Director
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Common stock reserved for issuance to non-employee directors	557
Director | 2011 Incentive Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Common stock reserved for issuance to non-employee directors	100,000
Maximum
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
No of installments for payments upon termination	Payments upon termination will be made in either one lump sum or up to five annual installments, as elected by the eligible director at the time of the deferral election
Pre-tax basis
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Maximum annual employee contribution, percent	50.00%
After-tax basis
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Maximum annual employee contribution, percent	10.00%