Debt - Interest Rate Swaps (Details) - Cash flow hedging - Designated as hedging instrument - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Interest rate swap - May 2019 - 1
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.57%
Notional value	$ 100,000,000	$ 0
Interest rate swap - May 2019 - 2
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.57%
Notional value	$ 62,500,000	0
Interest rate swap - May 2019 - 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.57%
Notional value	$ 15,000,000	0
Interest rate swap - January 2020 - 1
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.63%
Notional value	$ 50,000,000	50,000,000
Interest rate swap - January 2020 - 2
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.63%
Notional value	$ 50,000,000	50,000,000
Interest rate swap - January 2020 - 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.46%
Notional value	$ 50,000,000	50,000,000
Interest rate swap - January 2020 - 4
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.46%
Notional value	$ 50,000,000	50,000,000
Interest rate swap - January 2020 - 5
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.66%
Notional value	$ 50,000,000	50,000,000
Interest rate swap - January 2020 - 6
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.66%
Notional value	$ 50,000,000	50,000,000
Interest rate swap - January 2021 - 1
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.74%
Notional value	$ 75,000,000	75,000,000
Interest rate swap - January 2021 - 2
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.75%
Notional value	$ 50,000,000	50,000,000
Interest rate swap - January 2021 - 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.53%
Notional value	$ 37,500,000	37,500,000
Interest rate swap - January 2021 - 4
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.53%
Notional value	$ 37,500,000	37,500,000
Interest rate swap - January 2021 - 5
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.46%
Notional value	$ 100,000,000	0
Interest rate swap - January 2021 - 6
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.47%
Notional value	$ 47,500,000	0
Interest rate swap - January 2021 - 7
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.47%
Notional value	$ 47,500,000	0
Interest rate swap - January 2021 - 8
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.47%
Notional value	$ 47,500,000	0
Interest rate swap - January 2021 - 9
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.47%
Notional value	$ 47,500,000	0
Interest rate swap - October 2021 - 1
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.60%
Notional value	$ 55,000,000	0
Interest rate swap - October 2021 - 2
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.60%
Notional value	$ 55,000,000	0
Interest rate swap - January 2022 - 1
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.78%
Notional value	$ 100,000,000	0
Interest rate swap - January 2022 - 2
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.78%
Notional value	$ 50,000,000	0
Interest rate swap - January 2022 - 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.79%
Notional value	$ 30,000,000	0
Interest rate swap - April 2022 - 1
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.68%
Notional value	$ 25,000,000	25,000,000
Interest rate swap - April 2022 - 2
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.68%
Notional value	$ 25,000,000	25,000,000
Interest rate swap - April 2022 - 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.64%
Notional value	$ 25,000,000	25,000,000
Interest rate swap - April 2022 - 4
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	1.64%
Notional value	$ 25,000,000	25,000,000
Interest rate swap - January 2024 - 1
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.60%
Notional value	$ 75,000,000	0
Interest rate swap - January 2024 - 2
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.60%
Notional value	$ 50,000,000	0
Interest rate swap - January 2024 - 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.60%
Notional value	$ 25,000,000	0
Interest rate swap - January 2024 - 4
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.60%
Notional value	$ 75,000,000	0
Interest rate swap - January 2024 - 5
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Interest rate	2.60%
Notional value	$ 75,000,000	$ 0