Derivative Financial Instruments - Floating-to-fixed interest rate swap agreements (Details)	Dec. 31, 2015 USD ($) swap	Dec. 31, 2014 derivative
Derivative [Line Items]
Notional	$ 400,000,000
Interest Rate Swap
Derivative [Line Items]
Number of interest rate derivatives held | derivative		5
Interest Rate Swap | Notional amount of $100 million
Derivative [Line Items]
Number of interest rate derivatives held | swap	3
Interest Rate Swap | Notional amount of $50 million
Derivative [Line Items]
Number of interest rate derivatives held | swap	2
Interest Rate Swap | Notional amount to mature August 2016
Derivative [Line Items]
Notional	$ 100,000,000
Fair Value	$ (200,000)
Pay Fixed	0.87%
Receive Floating	0.42%
Interest Rate Swap | Notional amount to mature August 2017
Derivative [Line Items]
Notional	$ 100,000,000
Fair Value	$ (668,000)
Pay Fixed	1.29%
Receive Floating	0.42%
Interest Rate Swap | Notional amount to mature August 2018
Derivative [Line Items]
Notional	$ 100,000,000
Fair Value	$ (1,441,000)
Pay Fixed	1.69%
Receive Floating	0.42%
Interest Rate Swap | Notional amount to mature November 2016
Derivative [Line Items]
Notional	$ 50,000,000
Fair Value	$ 9,000
Pay Fixed	0.65%
Receive Floating	0.42%
Interest Rate Swap | Notional amount to mature November 2017
Derivative [Line Items]
Notional	$ 50,000,000
Fair Value	$ (139,000)
Pay Fixed	1.10%
Receive Floating	0.42%