Share-Based Compensation - Summary of Liability Based Award Activity (Details) (Liability Based Awards, USD $)	9 Months Ended
	Sep. 30, 2014
Performance unit incentive plan (PUP)
Summary of Liability-Based Award Activity [Roll Forward]
Beginning Balance, Shares	299,768
Granted, Shares	123,300
Vested, Shares	(94,600)
Forfeited or Cancelled, Shares	(7,800)
Ending Balance, Shares	320,668
Beginning Balance, Weighted-Average Grant Date Fair Value	$ 23.46
Granted, Weighted-Average Grant Date Fair Value	$ 23.71
Vested, Weighted-Average Grant Date Fair Value	$ 23.01
Forfeited or Cancelled, Weighted-Average Grant Date Fair Value	$ 24.93
Ending Balance, Weighted-Average Grant Date Fair Value	$ 23.66
Restricted Stock (RSU)
Summary of Liability-Based Award Activity [Roll Forward]
Beginning Balance, Shares	430,899
Granted, Shares	89,800
Vested, Shares	(137,187)
Forfeited or Cancelled, Shares	(17,840)
Ending Balance, Shares	365,672
Beginning Balance, Weighted-Average Grant Date Fair Value	$ 22.78
Granted, Weighted-Average Grant Date Fair Value	$ 23.81
Vested, Weighted-Average Grant Date Fair Value	$ 22.59
Forfeited or Cancelled, Weighted-Average Grant Date Fair Value	$ 22.13
Ending Balance, Weighted-Average Grant Date Fair Value	$ 23.14
Performance Based Restricted Stock Shares (PBRSU)
Summary of Liability-Based Award Activity [Roll Forward]
Beginning Balance, Shares	28,560
Granted, Shares	6,700
Vested, Shares	(9,890)
Forfeited or Cancelled, Shares	(500)
Ending Balance, Shares	24,870
Beginning Balance, Weighted-Average Grant Date Fair Value	$ 22.91
Granted, Weighted-Average Grant Date Fair Value	$ 24.95
Vested, Weighted-Average Grant Date Fair Value	$ 23.45
Forfeited or Cancelled, Weighted-Average Grant Date Fair Value	$ 27.32
Ending Balance, Weighted-Average Grant Date Fair Value	$ 23.16